The trouble with the minigames is that many of them are repeats from previous series, simply dressed up with a different scenario. There are a few gems in there though, as well as several nightmares.
I hated Slot Car Derby. Don't know what is was but the controls just seemed off to me and it wasn't very interesting at all. Most of the N64 ones were good otherwise, including but not limited too: Bumper Balls, Bombs Away!, Face Lift, Hexagon Havoc(?), Platform Peril and Crazy Cutter.
Mario Party 4 was mostly good for it's team games. I liked Tree Stomp, Hop Or Pop, Goooooooal!! and Blame it on the Crane for 1 vs 3, and enjoyed Cliffhangers, Team Treasure Trek, Order Up and Dungeon Duos for 2 vs 2. I hated Right Oar Left? with a passion and still have it turned off in custom mode.
I can't recall any real standouts of series 5 through 8, but I always hated the pure luck Battle mini games. So very annoying >_>
Now that I think about it, how long has it been since number 8 was realeased? Is the series finished?