My son loves anything mario, he's 6 almost 7. Super mario party, super mario bros...but he also likes the lego ones...batman, indiana jones, star wars...
are the lego ones 4 players?
They LOVE the New Super Mario and the Mario Olympics. I think you can't go wrong with Mario. 

It absolutely is, but my 5yo gets pretty frustrated with it at times. If we were talking 7,8,9 - I'd say definitely yes! But if there's an under-5 crowd, probably not IMHO.Mario Cart is so much fun!
Hands down the best game, New Super Mario. Came out on Wii in November last year and it plays for a long time. We got it for Christmas and both my 3 and 5 year olds love playing it. We now have four remotes and all play together.
We use to play mario party 8 as a four player...2 teams of 2. The boys loved it when they were younger.