My opinion on the one's I've played...
MP2: It's alright for the first sequel. It had really good minigames IMO.
MP4: THe presentation was overall nice I have to say, and the mingames were good but my opinion is biased to the fact that this was my first GCN game a long time ago so of course I'm going to like it...
MP5: I don't hate it, but at least they tried to innovate...even though they failed. It also has this weird vibe to it.
MP6: I don't really remember besides the day/night thing and the microphone thing which seemed cool at the time.
MP7: I haven't spend too much time with this either before my disc cracked. But unlockable characters and stuff was cool. I didn't like the new ways of getting stars though...
MPDS: Not bad for it being on the DS and still being better then some of the console versions.
Honestly, they should just make a huge Mario Party game with a huge story/scenario and basically do what Brawl did (or Melee, actually) to the SSB series.