SF II for SNES - I invested a crazy amount of time into this game - I'll tell you how much I rocked playing this game: I was able finish the game using every character (even Zangief) on the hardest difficulty without continuing - anyone who's attempted to do that will tell you it takes some serious skills. I kinda wish online play was available back then, I would have owned all.
Tekken for PS1 - another favorite that I invested a crazy amount of time playing. I loved destroying my mates playing this game... was funny watching their faces as I 10 hit combo'd their arses.
Its hard to think of the worst fighting game but I'm giving that title to Pitfighter - it sucked.