I really enjoyed Unison actually, though as a "party game", it obviously wasn't even in the same league as Bust a Groove, with you having to actually "learn the steps" to be effective at any of the songs. I still pull out Unison once every couple of months though, and still have a damn good time
Speaking of party games, and since I don't remember seeing it mentioned here as "essential", I think the PSX game that still dominates the most of my time, especially when people come over, is Super Puzzle Fighter II Turbo. In my mind, still the best puzzle game ever created, save maybe the original Tetris on linked Gameboys
