1, easily. A simply amazing game that raised the bar to 11 for the genre. Everything about it is perfect; the graphics, the music, everything...not only that, but it's a wonderful satire on war, and when you beat the game, you finally see what it's all about. Incredibly poignant.
X for me. They got the difficulty just right with this one. I dont like metal slug 1 very much becasue its too easy up until the middle of the last stage, its almost a chore to play through.
I love Metal Slug 3! The other Metal Slug-games are quite cool, too, but the 3rd time you shoot funny soldiers is the most impressive one, in my opinion. There so many neat animations details that every new run is a new adventure.
I played the first Metal Slug in a Cabinet and if I should compare MS 1 and 3, I would choose 3 played on a cabinet screen, even if it already was a magnificent feeling to play ms1 on that big "show me every single pixel"-screen.
Definitely the first one for me. Worth half an hour of anyone's time. The others are fantastic (I even think the 4th one is excellent), but I find the original the most enoyable to play. It has a really well-balanced level of difficulty (unlike MS3, for example) making it a fun one to pick up and play any time, and it's easily the most thematically consistent... which may be unimportant in the run 'n' gun genre, but it definitely feels more "complete" than the others, and I'd suggest that this is partly why.
It's just a well designed game with the intent of playing through on a single credit. Not too long, not too short, and just right.
MSX is awesome, just something about it that's not quite perfect. MS3 is epic as hell, but that's just what kills it for me. Always fun to co-op and quarter feed though.
Haven't spent enough time with MS5 honestly. I just don't enjoy it on Xbox. Need to buy the MVS cart.