I personally love Mario Party games, but thought 4 was the worst, i recently played 5 though, and it's already my favourite Mario Party game, i love it, it's heaps better than 4. Soul Calibur 2 is one of the best multiplayer games, i love SC2. Oh well.
Personally, i havent really played many crappy games, so the worst game i've played is Rogue Leader. I cant stand the gameplay, sure it has awesome graphics, but i hate it.