I rather enjoyed Bushido Blade myself, because of the realistic factor, i.e. a sword to the neck=death.

Super Smash Bros. Melee is the best. Hands down.

I just got Soul Calibur 2 and I must say I'm somewhat addicted, although that's so that my friend won't beat me when she comes over. I tend to use Kilik or Link (I wanted the Gamecube port because I don't like Heihachi).

I like Marvel vs. Capcom 2 as well, although I'm not that good at it.

Mortal Kombat, quite frankly, doesn't appeal to me. The story, as someone already mentioned, is far too overdone and not that compelling. My cousin just got Deception, we sat there for maybe 30 minutes playing it and seeing all the fatalities. After that, we got bored and moved on.