6 was my first, so by default it is my favorite.
I actually enjoyed all the Gamecube Mario Parties in one way or another. The first one on the 64 was horrid and way more unfair than the others. 8 was decent but disappointing, never played 9.
Also, I ended up getting Mario Party 10 despite my rants and stance against it. It's okay but about as bad as I anticipated sans Bowser Party mode.