The argument about which top-class Mario game is better rages on. =P
Galaxy 2 still remains as my favorite 3D Mario, but 3D World still sits as my 3rd fav after Galaxy 1. Though I'd really much rather enjoy than pick-and-choose.
I agree 3D World eloquently merged 2D and 3D better than the others. I do ultimately prefer 3D World's more unrestricted use of items, and its more free-roaming World Map. It's 8-way movement and run button wasn't an issue, but it's not what I'd prefer.
However, no Mario title has filled me with much whimsical platforming bliss like the Galaxies, particularly 2. I don't know, it felt more fluid to me. There emphasis on gravity, the surreal and "out-there", and there soundtracks (both are on equal terms, IMO) put it above the others, though I do think the jazz emphasis fits Mario better. Even though Galaxy 1's majesty was lovely, the sequel's whole shtick of being weirder and more playful in its concepts and worlds made it a more fun experience.