IK+ is the best beat 'em up ever made, seriously.
OK, I guess it depends very much on what you want from your fighting games, but what I tend to want is fast, fun gameplay, simple controls that still offer a lot of options, a reason to actually play the single-player mode (which is why no other fighter gets near this in the list), and a solid multiplayer mode.
One of the perennial classics of 16-bit gaming, IK+ blew me away when I first loaded it up on the Atari ST. Countless hours were spent in winner-stays-on tournaments, and I played the single-player mode extensively (level 3 black belt, right here). I even love the mini games that are thrown in. The animation still looks great, the challenge compulsive and an 8-way joystick and a single fire button provides all the moves you could want.
There's such a fun sensibility to the game, from the little codes you can type in for various effects to the obvious kick programmer Archer Maclean got out of the presentation. It's effortlessly cool. The Amiga version takes the prize for the fantastic theme tune and nicer effects, but there's nothing in it in gameplay terms between the rival 16-bit computers.