I love Mario Party 9 and it might in fact be my favourite, I understand why it's unpopular but I think it's incredibly funny and enjoyable. Though I have a soft spot for Mario Party DS too.
Without a doubt, 6 is my favorite. It was the first Mario Party I owned, so I really got the chance to dive into everything it has to offer. I love the day/night mechanic, and the orbs feel so much more refined here than they are in 5. The boards offer some great variety as well. I wish that Mario Party Superstars picked some better minigames from 6, because it's a solid lineup there too although Trap Ease Artist does indeed suck. There's even a goofy game show mode with the Mic! I could go on but that's all of the major points I'd say.
Before Superstars came out, I considered Mario Party 10 to be my favorite because of all the good memories I had playing that game. Nowadays, I won't play anything other than Superstars and maybe Super (only with other people though) because they're the most recent ones...
i have a lot of memories of MPDS, MP9, and MP7, so those are my top 3 favs for sure, especially 7, which is actually my favorite Mario game/game in general of all time
I think MP10 gets too much hate, the 3DS/WiiU Era was very underrated and had a lot of classics in there, sure there were some stinkers (f___ Ultra Smash tbh) but still awesome