I've put an insane amount of time into Mario Party 4, most of which was spent on only two boards. This game would be the Mario Party formula perfected if not for the fact that most of the boards kinda suck for one reason or another. There's some fun stuff in there, but the real party lies in the minigames. Most of the minigames are fun and highly replayable. Plus, the ability to remove minigames from a custom list is one of the best ideas a game like this has ever had... yet they never revisited this idea.
Mario Party 4 is really close to being my favorite Mario Party game. It's also probably my favorite from the GameCube era.