Best: PM: TTYD. This one kept me interested for the entire game, even when the action got repetitive.
Worst: Mario Party 6. The sun and the moon are fighting over who's better, and then Mario suggests that they have a party in order to settle the feud. [wtd]? How is that supposed to stop their fighting? Better yet, if the sun and the moon are fighting, why are they seen cooperating during the game? This game was the worst Mario Party and it had the worst plot, too.