Easily Super Mario Galaxy.  That game just had such great level design, memorable locations, fantastic gameplay, and the element of worldbuilding (by that I mean improving the hubworld and stuff like that, which really makes you care about the objective, not minecraft worldbuilding) with what's at stake surpasses all the others.  It also has a ton of spirit put into it, which is why I like it more than smg2.  I like world and 3 for what they are, but objectively, Mario Galaxy is just so much better.