Mario Kart, Mario Party and Mario Baseball. I like Soccer, but Strikers's cheap AI is annoying.
If I had to choose only one, either Mario Party 6, Mario Party 7 or Mario Super Sluggers.
6: Interesting minigames.
7: 8 Player support. I don't have 7 other players and I have only 2 Gamecube controllers, but I like having lots of opponents.
Sluggers: 4 Player support during baseball matches and 2 vs 2 support in Toy Field.