The PS2 games I've had the most non-splitscreen multi-tap fun with are Mashed, Urban Reign, and Def Jam Fight For New York.
http://en.wikipedia.org/wiki/Mashed
http://en.wikipedia.org/wiki/Urban_Reign
http://en.wikipedia.org/wiki/Def_Jam:_Fight_for_NY
Also for sheer party fun, don't rule out the EyeToy and the games EyeToy Play and EyeToy Sega Superstars... I've quite possibly had more multi-player party fun with the EyeToy, friends and "inebriation" than any other game I've played.