Oh interesting, an SNK-themed sale. I hope we might see a few more SNK titles, they were on a nice run for a while there with most of Obada's topdown rotary run/guns in the bag. WTB Search And Rescue, currently only on console in semi-jank form via SNK40th. (SAR is so surpassingly excellent, I don't regret having SNK40th around, but its quality is dire next to ACA)
SHOCK TROOPERS is absolutely and utterly essential. Among the best topdown run-and-guns ever. Super smooth controls, tons of varied characters and stages, all of the above mix-matchable via Team Battle and route select. Bangin choons too.
NEO TURF MASTERS (that's actually the localised title, no longer in use due to legal disputes with the Augusta Masters golf tournament) is the kind of arcade sports game so good, it'll very likely hook you even if you've absolutely zero interest in the sport itself. It did me! Super compelling just-one-more-credit factor, with lovably goofy Nazca charm and first-rate HIYA tunes. Goes by its (much cuter imo) JP title
Big Tournament Golf nowadays.
I've always been intrigued by
ASO II, does some unexpected things for an arcade title. Happily picked it up on ACA pending a possible MVS cart.
Handy guidance from MJR here! KOF2002 is neck and neck with KOF98 for the series' best Neo entry, and some of the best 2D fighting full-stop. Fighting games demand company to play them with, but if that's not an issue, it's an evergreen favourite for good reason.
Metal Slug 5 is the weakest Neo Slug, though that's not too hard a knock considering their general excellence. Takes a bit too long to heat up, though it does deliver very good third and final stages, and a genuinely excellent fourth.
COMMANDOOO 
If you're new to the series, I'd suggest jumping in with MSX, probably the best-balanced overall. MS1 is just as likeable but a bit easy, MS3 is great too but infamously long and unforgiving; can't go wrong with X, which is both titanic, accessible and concise. Noise Factory's MS4 is best thought of as a mission pack / Official ROMhack; I actually put its action on par with Nazca's games, maybe even a bit better - it reaches their endgame difficulty earlier, where they tend to spike violently at the 11th hour - but it's nowhere as satisfying artistically, an undeniable selling point of the earlier games. Get it if you like them and want more+harder action.
(MS2 is wracked by utterly god-awful slowdown, something MSX hugely alleviated while adding a bunch of excellent new content - a curio these days, imo)
Ikari is an idiosyncratic classic. Simmering death-march intensity with lots of tactical improv. You're slow, but so is everything else, and your view of the battlefield is absolute. Only caveat is the
memo-reliant endgame, but it's a trip worth taking if you dig this mode of slow-burning action. I'd say give it a try in MAME first, but the rotary controls are absolutely boned there. Conversely, they're as good as they could possibly get for gamepad, in ACA. Here's my recommended setup:
Looks nightmarish, but works a charm imo. Use the manual turn buttons for precision aiming. Use the rapid ones when you need to quickturn (this is more of a thing in its sequel, Dogosoken, which is all about sword slashing). Use the right analogue when you need absolute point n' shoot speed, technically surpassing the OG rotary sticks.
Sasuke VS Commander - primordial gallery shooter with a bit of run/gun panache, and some of the genre's earliest boss battles. It's a fun time, though as always with games of this vintage, YMMV. Does one thing and does it very well.
For a non-rec,
Athena is a genuinely poor game, unfortunately. Busted fundamentals, annoying structure, lacking presentation... absolutely no match for its fellow 1986 SNK icon Ikari. Definitely try it out in MAME first, if anything. This game singlehandedly taught me that even as cheap, space-efficient digital purchases, bad games hurt my curator soul.
For 1986, forget about this one and grab
Argus no Senshi aka
Rygar instead, a classic whose STGesque precision and intensity leaves SNK's game and a good 90% of other sidescrolling action games standing.
I'm a Pac-Man casual at best tbh, but it's always nice to know exactly which revision you're getting with such foundational games.
I'd definitely suggest MSX next if you like MS1, it's a near-perfect sequel. I love MS3, but it tends to sharply divide opinion with its infamously overlong endgame. As for the stuff listed there, the single best is probably Shock Troopers; just like Metal Slug, it's got airtight fundamentals, rampant destruction, and a wicked sense of humour, plus the Team Battle mixup is just superb; I wish more run/guns had experimented with it. Neo Turf is absolutely first-rate too, just not quite as surefire universal.
Bulk up in the off season, trim down for the big show! 
I had to go with the totes sus normal mode for my 1LCs of both. The former feels claustrophobic, and screws with my positioning - the latter is just gross. No spraying cum in my red-blooded run/guns please. 
My only complaint is seeing this running in 