I would have to agree with Thrill Kill. It has to be my favorite leak of any game ever made. I even was able to find all 4 versions of the game. For the record the Thrill Kill Engine was used in Wu Tang, both Xmen Mutant Academys and Rock'em Sock'em Robots Arena. Of course in some of the games it was cut down to a 2 player game.
For me it would have to be the russian half 2 beta, though not really playable. I have heard that some people made a mod which corrected that. Not sure what it is called though.
First, would have to be the Sonic X-Treme 718 release, second best, would have to be Virtua Hamster which was released yesterday.