Mario Party 2 would get my vote because of the themed costumes, board design and overall great mini games.

Best one I seen was an unofficial one

Going to have to go with Mario Party 2. Played it a ton when I was a kid.

I loved 6, but it is 99% based on nostalgia probably.

Mario Party DS is one I hold near and dear to my heart

2 or 3, with a personal, slight edge to 2.

2 was my fav, but I haven't played 3 in so many years, and 4 and 5 are actually pretty decent.

8 is the only ones my friends can actually beat me in x )

I liked MP2's music better because Yasunori Mitsuda (of Chrono Trigger Fame) composed the soundtrack. Overall MP3 wins though. Had a ton of fun with 4 as well.

Mario Party 5. It brought in the capsule items and the mini games were great.

Mario Party 2 and 4 are my favorites.