I voted for SMG1, SM64, and Luigi's Mansion (though I don't think this one should count as it's a spinoff, but whatever).
SM64 still holds up really well over the years. I replay it on the VC from time to time.
SMG1 was also very fun. One of the best games I have played of all time. I have played through it at least 4 times since I got it in April of 08. I must admit I haven't played it much or at all since I got SMG2. The problems I had with SMG2 was
-too many minigames (more platforming and less kill certain amount of enemies in certain time limit)
-Green Stars (they just weren't very fun to collect to me)
-more galaxies but less stars to collect in them (they have 1 or 2 "regular" stars in each galaxy, I was kinda dissapointed)
-Too much reused stuff (6 bosses completly unchanged, a LOT of reused music from the first Galaxy, A couple of rehashed stages (2 from SMG and 1 from SM64)
-Less Epic (the opening/ending weren't as good IMO and the Bowser battles weren't as fun to me)
-No HUB (
I love to goof around in HUB worlds. Starship Mario is just too small. granted SMG's HUB was weak and small but I still found fun in using Fly Mario, and jumping down from the top of the obsevatory)
and for those reasons that is why I didn't enjoy SMG2 as much.
Luigi's Mansion is a great game and wish they would make a sequal.