Quick Note: hyiu - The Metal Slug games are unique separate games (except X and 2 which I've combined), so anyone who wants to vote for these games should vote on them separately. If you love all 3, then vote for all three. For your top 20 I made MS1 3rd, MS2/X 4th, and MS3 5th and slid every other game down two slots.
On the other hand, I feel that the SFII games are just updates or tweaks of other versions, so the SFII games are counted as the same game. My logic is this: I want someone to be able to grab this top 100 list and just plug the 100 games in. In the case of the Metal Slug games, I feel they are unique enough that someone may want all three. However, in the case of the SFII games I feel someone is more likely to just find the best of them and play that game, probably never playing the other versions.