>>7177974504 is hard carried by having arguably the best minigame lineup in the series, the items and especially the board design is a massive step down from 3. duel minigames are also gone, and the game's pacing is slowed down to a crawl (something that gecko codes or MP4DX fixes.)
5 shakes up the formula with the capsule system, but none of the boards or minigames are "best of mario party" material. in fact some of it's minigames are contenders for the worst in the series. some people like 5, some people hate 5, it's entirely preference. 5 also brings back duel minigames which were absent from 4, but now you can wager stars.
6 speeds up the pacing while making the capsule system (now called orbs) more like traditional items. minigame lineup isn't as good as 4, but there's a few standout boards and minigames in 6 that put it in talks for the best mario party in the series. 6 makes duels their own space, with the same wagering rules as in 5.
7 is more like 6, but the pacing's worse, there's character specific orbs now, and the mic gets it's own space for a piss easy minigame that doubles your wagered money if you win. duel minigames are no longer luck based, but your reward for winning them is, and sometimes you can just roll a dud, making the duel a complete waste of everyone's time. Boards are mostly fine, but Neon Heights tends to be a fan favorite.
tl;dr the best gamecube mario party is the one you grew up with. if you grew up with none of them, 6 is the fan favorite right now.