I'd have to say Tekken has the best combat system. Killer Instinct was pretty bad ass, but I haven't played it in many years, since my n64 died, I wish they came out with new ones. Mortal Kombat has some really cool characters, but there are so many of them. Armegeddon is the best of all MKs because of the variety.
Heihachi could wreck anyone in any of these games.