Unfortunately, I've only played Mario Part Island Tour. So I guess I kinda have to say that. But from what I've seen on Game Grumps, Mario Party 4 looks like a Hell of a good time.
MP3 is my favorite. 1 & 2 are close by though.
MPIT has been slammed a little hard, I've been playing it by myself and with friends and both seem enjoyable...
Probably 5, 6, and 8.
5 had the car arena or whatever it's called.
6 had the day/night cycle which made it interesting. It also had the best use of the Microphone.
8... was just fun. No good reason.