I've only played it for one night of course, but from my initial couple of 25-turn games (and some single player action) I'd put it like this:
MP1 > MP6 > MP5 > MP2 > MP4 > MP3
I still think the original is unmatched in many ways, but so far MP6 is proving to be very nice and has quite a few new ideas thrown in. For example, we just played on a board where there are no star-buying spaces. Rather, you all start with 5 stars, and the only way to steal them from each other is to buy a ride on Chain Chomps that're scattered around the board and run over your opponents. To mix things up more, the paths you can take to avoid the line-of-Chain-Chomp-fire areas are blocked off for 3 turns every 3 turns, when the board turns to night, which forces you to get near them. It's a constant battle to get behind the other players and take them out.
Another interesting thing - the "had the most coins at any given time" star normally awarded at the end of the game is gone now, and has been replaced by a "used the most items" star. Kinda cool, it encourages you to really go nuts with the items.
These are just a couple of things off the top of my head, but there really are a lot of changes to the usual formula mixed in this time. Some subtle, some pretty huge. The mini-games are some of the best yet, also. I don't know what happened for a while there, but 5 and 6 have really come back in full force with possibly the best mini-games in the series. Still no old-skool analog twirling games, though, and only 6 maps (this includes the unlockable one) compared to MP1's 8 maps. Boo!