Mario Party 8 had great visuals. I don't know why everyone is like, "hurr early gaemcoob grafix AT BEST". It was a beautiful game (the mini-games, especially). I liked the depth of field going on... you know, how it'd blur everything in the distance upon a close-up of a character on the board; however, it was a kind of ugly blur, if I remember correctly. The only horribly disappointing thing was those ugly 4:3 pillarboxes. When I rented it I desperately looked for an option to get rid of those stupid things, but of course, no such option exists. Sad...
As for best Mario Party games, I'd say 1, 2, 3, and 4. 1-3 had awesome mini-games and 4 was just like, "holy crap, gamecube mario party? they're steppin' it up with the graphics and mini-games here." I didn't even mind that the board was floating above the terrain. It gave it a good feel that way. (I also love the updated die roulette sound from the previous Mario party games and the fact that the camera angle was set lower so you could see most of the map instead of Mario Party 3's almost helicopter-filming-a-high-speed-chase aerial view).
1-4 were great, 5 introduced the fall of items in favor of those stupid 50 cent gumball machine plastic orb "items", 6 was decent (the microphone was more of a poorly-executed gimmick than current Wii shovelware), I don't remember playing 7, and I just couldn't stand the pillarboxes in 8. It really killed the immersion; it felt like I was playing the game while looking through a wooden fence.
With Nintendo being so lazy and recycling everything these days, I'm actually extremely surprised they haven't revisited the older mini-games/boards from times past. If they were ever to do that, I figure they'd compile the crappiest boards and mini-games from the newer games and probably re-badge the majority of Mario Party 8 calling it: "Mario Party 9: Mario Party 8 MotionPlus New Play Control!"
What they should do is update all the old games and have it be a "Mario Party All-Stars" kind of thing including Mario Party 1-4 all updated graphically, etc.