Oh god this is rather tough for me... I can't stand 5, 7, 8 or DS. -__-
5 had really good mini-games, but TERRIBLE boards. The boards were just dull and capsules... Oh god don't get me started. >__>
7... Oh no. Oh no. The thing that REALLY breaks it for me is the F***ING BOWSER TIME. Terrible mechanic. We already face trouble when we land on his space, but every 5 turns? No thanks. On top of that what he does is and random and you will lose coins/stars or have something horrible happen, like preventing you from going in an Orb Shop. Oh god crushing your windmill in Windmillville was a TERRIBLE THING. Plus it's a waste of time and makes boards take FOREVER.
Not only that but Orbs were nerfed from MP6 AND mini-games are lacking in quantity and quality. (For real only 13 4 Player mini-games? NO WAY.)
8... Slooooooooow gameplay. Mini-games were lacking in quality and boards were terrible. Well actually I love Shy Guy's Perplex Express.
DS was just super bland, items were weird and inexpensive and the boards are small. At least it had decent mini-games.
I don't count Advance. tbh though I actually kinda found it decent as a game.