Galaxy is the prettiest game that I've ever played that has little substance in terms of gameplay. I genuinely think that it's the most aesthetically pleasing game I've ever played - that or NiGHTS Into Dreams... Most of the stars were really unmemorable and dull, though. Luigi's long jump totally breaks the game on the second playthrough, too. Some of the stuff that I actually died a few times on in the Mario playthrough, like Toy Time's Purple Coin, I found really easy to do because of that long jump.
I actually haven't really enjoyed a Mario 3D platformer since Mario 64. I thought that Sunshine was really padded with the blue coins, Galaxy 2 had really poor stars (especially some of those Green Stars), and SM3DW was just really dull. The complaints that people have about Rayman Legends - it was designed for multiplayer and thus the level design was not as tight - I think are really more appropriate for SM3DW. It's just really dull and having to collect all the things that you need to collect to get to Champion's Road is not worth it.
I haven't enjoyed a 2D Mario lately either, though I have not played SM3DL since I don't own a 3DS.
Mario is the Jeb Bush of franchises at this point - big name that never lives up to its potential.