I balloted for II (AC), III (AC) and Gaiden. I was seriously considering III (SFC) instead of its arcade original, I love both for very different reasons (see below). It also should be stressed that I would like to clear at least two loops of III (AC), IV and V as well as many loops of ReBirth before I comfortably feel able to expound a profound verdict, looping is an important matter for me in this series. It's unlikely I'll ever play some of the more peripheral games - even the improved version of 2 (MSX) felt extremely unnerving to me, and even normal people can clear several loops of the GB games which I find suspicious.
Here's my personal overview/wall of text on the rest (keep in mind that even my least favourite Gradius game is far above almost everything else, those are merely comparative statements):
- The first Gradius is an exuberant pleasure to play - it probably embodies the improvisational, instinctive action better than any other game in the series. Positioning the options and methodically working with the stages is not nearly as important as it is in later games, most enemies are usually out in the open and easy to hit - double has the appropriate angle, laser has a massive hitbox that seems to grow even larger if you move around a bit. Since the hitbox of the ship is quite huge and the shield is even bigger, you're forced to tank a lot of hits on later loops instead of manipulating suicide bullets or shrewdly taking enemies out at opportune times. It's unfortunate that the checkpoint in the middle of stage 4 exists and that the hitbox couldn't be a tiny bit smaller, I don't see the need of a run-killing recovery in this first game. Dying on later loops should entail grave consequences, but this particular checkpoint feels completely out of place, it's easily as hard as some of the nastier counterparts in II & III. As such, I think the more lenient recovery difficulty in the PCE and Famicom port are a welcome change, both are superb conversions. Interestingly enough, despite (no: because) its shortcomings for technical reasons, the Famicom version is closest to later Gradius games in that you might want to play with a lot of speed-ups in order to space the options apart since you have only two of them. This gives an interesting contrast to the aforementioned scoring-oriented changes. The PCE port has the worst slowdowns out of the bunch, the MSX-themed stage is superb, however, and it maintains the higher difficulty of the arcade game for the most parts while making recoveries a lot easier. I'm accustomed to the faster and more numerous suicide bullets of the arcade game now, but it's nonetheless a very balanced experience especially considering that the difficulty upon looping rises linearly instead of exponentially. Having to fight the Big Core so many times is a bit of a shame although understandable for the time.
- I love II, it's my personal favourite. It's the first game in the series to have a decided focus on stage conception that is centered around smart option utilization which makes seemingly small decisions (number of speed-ups; playing with three options vs. four, the latter of which causes the option thief to spawn) crucial. Higher loops completely alter how stages 1 & 4 play out (stage 4 even has a brand-new layout) and require careful planning to not get overwhelmed without ever dropping the instantaneous nature of dodging (suicide) bullets - a wondrous high-wire act! Being able to use some quirkier strategies (like not shooting little moai heads or zubs or refraining from fire to influence the behaviour of duckers) for easier survival is a great addition, II is also the game in the main series where you can score quite a bit more or less per loop depending on precise/risky you play which is also an excellent incentive to practice/study earlier parts. Checkpoint recoveries are usually very manageable even on high loops, the fourth stage and one checkpoint of the final stage are very unfortunate exceptions. They're doable after a lot of practice and knowledge, yet especially stage four is needlessly difficult to recover from for all the wrong reasons. There should never be any checkpoint without a power-up, that's just bad. Chances are your run is over if you happen to die there after several loops due to weariness. The PCE port is marvellous for the most parts, after being accustomed to the arcade game so much I'm having some difficulties adapting to its higher loops, though. Nonetheless a stellar effort, it really is a stupor mundi. The new stage is outstanding and pronounces the established prudent option placement in an exemplary manner. I'm not a huge fan of the Famicom version - it's also a little marvel in its own right, but it never gets more difficult which is understandable (I'm going to assume that clouds of suicide bullets weren't feasible on the system, lest not without massive bouts of flicker, the first Famicom conversion also omits those) yet still a bit boring. Due to the semi-random amount of points you get out of Club and the scoring exploit on the Moai boss counterstopping/high looping becomes even more futile. It's definitely a much better game if you just want to have a well-crafted one-loop experience.
- Shepardus insinuated it in the other thread: III (AC) had all the potential to be the pinnacle of its series. And I want to say it does have some of the finest moments within the series, there's no denying it also has some of the biggest issues on the other hand. I absolutely think that people are generally way too dismissive towards the game, I can regrettably understand why that is, though. If you beat the SFC port on the arcade difficulty the game will state you're a "technical monkey" which is an apt description for those who'll get a lot of enjoyment out of the arcade game, incidentally. Option placing and spacing is even more important here than it was in II which is an enormous accomplishment. Combine that with an expertly crafted pair of double/laser (playing with Type B) and you'll have a blithe time figuring out some of the amazing stages - I particularly want to mention the moai, cell and most importantly the fortress stage, but a lot of the other stages are grand, too. Learning how to safely dispatch hatches and moving around shifting walls in the fortress was some of the most fun I had with the genre. That also includes getting very familiar with a lot of the peculiarities of the game, such as some of the more questionable/goofy hit detections, which floors will only neutralize your shield and when the slowdown will kick in/abruptly subside. You probably must love the game before even getting semi-competent at it to get the most out of it - a lot of people will quit long before that and I don't blame them. Those are clear objective flaws that shouldn't be trivialized. I can luckily live with most of them myself. It also helps that I adore the music in the game even more than the soundtracks of its brethren which, again, is high praise. My only real issue is just how cruel the looping "conditions" are. Having to deal with temperamental regenerating cell walls and floaty brains which have criminally large hitboxes before getting to the second loop might be my least favourite design decision in the series.
You also have to carefully select your options before that in order to prevent the option hunter from spawning during that section... I'm still very excited to learn at least the second loop, it shall be a great adventure! I love the SFC port for its opposing qualities: I can just play it as I would play the first Gradius, it's sheer excitement and bliss. Being able to enjoy the outstanding audio-visual presentation and strong level design without having to concentrate on every move can be heaven sometimes. The only real critique I have is that recovering on the first and - even worse - the second boss on high loops borders on the impossible. Which means that high loop sessions/counterstops are frightening once you hit stage 1 and then become much more relaxing after stage 2 (although not downright easy!).
- IV is a lot better than its reputation. Having said that, I'm also one of the many players who is having difficulties with getting into it. The Gradius series always reused themes/ideas for stages, yet it never appeared to me as simple plagiarizing or recycling ideas. Some of IV's stages create the impression that the designers were shy of new concepts, though, or so it appears to me. The ice/bubble stage and high-speed stage are downright silly. That most likely also has to do with the odd music and weird visuals, it's also grossly unfair to discard those in a trice without acknowledging some of the ingenuity that went into them, I'm nonetheless having a hard time truly appreciating the game. Implementing the dynamic rank system was without a doubt a brilliant idea, the game is a lot more balanced than III as a result (it had the predisposition to be just as frustrating). IV also finds an enviable equilibrium of action and planning - the moai stage will require a lot of on-the-fly dodging/manoeuvering while the fortress stage is rather technical as usual etc. Several boss fights are quite great, arguably better than most in II & III. I'm also fairly certain that IV gains a lot from high loop play and I have to say that I'm eager to find out at some point. Shaking that awkward feeling is egregiously difficult which is a pity since IV does deserve better for sure. It doesn't help that certain sections genuinely emanate an annoying aura. That cell stage seems to be there only to aggravate players, I can't help it. I'm curious how much I change my opinion on the game upon getting better at it. It's perhaps the most repellent entry of the main series, maybe it's also one of the most rewarding ones once the initial hurdle has been overcome?
- V I respect for its bold changes and adaptations of the formula, I find it hard to love it, however. It's not quite the R-Type Leo of the Gradius series in terms of dissimilarities with the rest of the games, but there is a certain resemblance. While I have to spend more time with it to frame an even-keeled analysis I get the impression that it leans moreso towards the improvisational side of things than the technical, tactical aspect. This has to do with the small hitbox as well as the multitude of enemies and bullets which - at least from my experience - make it hard to meticulously plan out exact routes. That's not to say there isn't a lot of work put into methodical action, locking the options in place for example allows for some great strategies. I hope that I can feel more natural with the game at some point in time; I'm still not comfortable dodging more intense patterns, I miss the blazing-fast ship and I'm not as much of a fan of the majestic, grandiose atmosphere as opposed to the alert, adventurous spirit with a diverse set of levels with vibrant colours of earlier games. I also wonder whether this particular entry wouldn't have benefitted from a lack of infinite loops; due to the dense, epic scope of just one loop I could imagine it only having two or three loops instead of going on forever. That's only a theoretical reflection as of now, will have to find out in praxi.
- Gaiden is my second-favourite, closely behind II. It has the best boss fights in the series in my opinion, the most interesting diversions of ships/configurations, generally magnificent stages and arguably the most balanced checkpoint recoveries. I would go as far as to say it's objectively the best game in the series (V is too different to obtain this label, I reckon; a lot of what people love about V has to do with what it completely alters, not with what it keeps). The low extend rate perfectly balances the easier checkpoint recoveries to a point where it becomes arcade-difficult without ever explicitly feeling so. It's an uncannily natural game where all the set-pieces gaplessly come together. Thanks to the amazing boss fights there's also a great emphasis on aggressive, downright violent techniques which is tremendously satisfying by dint of the spectacular visual appeal. Stage gimmicks like the reflecting crystals or the gravitational pull of the black hole not only develop and independent life on a cinematic level, but are also stupendously dovetailed into the gameplay mechanics, again in a natural manner. Higher loops reward players for adding new dangerous layers to existing boss fights or by including the almighty Scary Core, also known as Heaven's Gate. All-around top-notch work, I can't think of anything to complain - both it and R-Type Delta were prime examples of how to gingerly adjust long-running series without turning them into something completely different.
- ReBirth might be a hidden gem amidst the series. I only 2-ALLed it with a couple of ships so far, but I have to say my first impressions are very benevolent. It's obviously a loving nod to the MSX series, yet that's not all there's to it. It reminds me quite a bit of the Famicom port of the first game in that it is unusually oriented on scoring. It's a short game (five stages) that gets even shorter when you enter the bonus stages (which will skip the rest of two of those five stages), it's not particularly difficult at first, but does add one more suicide bullets for every subsequent loop, you can earn extra lives and points by capsules, the option thief is gone etc. pp. You're definitely not going to get a fulfilling experience if you only want to tackle one loop, it's evidently construed around multi-loop sessions which is a substantial bonus in my book. Navigating through the bonus stages is a trifle hairy, familiarity will hopefully mend that. The throwbacks to other games are charming (before getting to the final boss you'll hear the mechanical "Warning" advice from Gradius III (AC), the moai boss perishes with a deafening Salamander Fire Dragon roar...) and I love the rearranged themes from the MSX games. Will gladly investigate this one more in the future, it's probably not going to land in my top 3, but it's also might not be too far from it either.