If I recall correctly, I loved every Mario Party game for the Gamecube. I don't even remember how many of them there were, though. I also don't know how much they cost. Couldn't you just emulate all of the MP games without paying anything? Then again, I completely understand the draw of owning the actual game rather than just a digital version.
Well you have 4 to choose from. I would recommend Mario Party 4 or 5. Both stayed true to the original structure, with just a few small twists added. 6 and 7 both have gimmicks with using a microphone for some of the mini games.
I personally like Mario Party 5 the best. Pushy Penguins is the best, and most frantic mini game of the whole series!
Really, all of the Mario Party games for the game cube are wonderful if you have friends to play it with. I can't say one is more fun or less fun than the other honestly. However, if you're looking for a nostalgia factor to add to the enjoyment, you could always try the earlier Mario Party games for the GameCube.