My Favourite is 3. There is so much good content in that game. Absolutely love the soundtrack (esp level 2) and all the animations are crazy. To this day I cannot beleive how much they crammed into this game, and the existance of elephant slug is like the level of work that went into Akira (you know where they painted a massive citiscape, only to use a small snippet in a handful of frames).
The 'Best' Slug however is 1. It is not too long, not to short, the diffiiculty is just right, and it looks just as good as the other contenders. The music also is probably my second fave.
3 - actually has too much in it, I think that had they pared down the last level it would have been the best. It's still my personal fave, but not objectively the best.
2 - is a lovely game, but the glitch that causes the excessive slowdown means that it just loses out to 1.
2>X of course.
4 - was my first home cart slug, and I liked it, but even though at that point I had never played the others very much (or 3 at all) it seemed like a re-hash. I think that I could still have fun with this game, but it is in a different category to 1,2,3.
5 - I have to be honest, I have hardly played this game, when I first played it I was really hyped by the beginning of the first level, I thought the initial jungle river section was as good as anything in 3 ... but it is clear that they didn't have the time to spend on it that they wanted, so the rest looks very generic. I would really love for 5 to be remade, with all the ideas and stage setups as they intended, that would really be a great successor to 3. Maybe call it Metal Slug Y?