I played SFII first in the Arcades (and was my first contact with SF franchise too), but the more nostalgic one to me is SFA2 on SNES (I just love that version, even being the most downgrade of all).... I played for ages, and still one of my favorites, though, in terms of gameplay, the PSone or Saturn ones are way too better (obviously).
And I forgot to mention about the Vs. Series, I didn't noticed that everybody is counting it too =P.
well, my favorite is MvC1... I love the simplicity, the roster, musics, the controls (that don't underestimates the player, without being as complex as SF games), the extras,...everything! ... I love :3
and my rank stay like that:
Marvel vs. Capcom - Clash of Super Heroes > Marvel Super Heroes vs. Street Fighter > Capcom vs. SNK 2 > Tatsunoko vs. Capcom (JPN and the western versions) > and the others.
(even that games like MvC2 and UMvC3, even with its faults, has a lot of potential and are still appreciated by the public and in large events).