I'm going to be different and say the Comet Observatory. The Comet Observatory is so calm and the music is absolutely soothing. The scenery is beautiful, the Lumas are as cute as always... it's just awesome. It's easily one of my favorite things about Super Mario Galaxy.
Isle Delfino is a close second, though. I've always loved Super Mario Sunshine (it ranks among my top 5 favorite video games), and Isle Delfino just gave the game the tropical feel that it needed. It's well-sized, like the Comet Observatory, the scenery is also nice; the NPCs are good as well, and the music is memorable and also gives the hub its tropical feel.
Princess Peach's Castle was nice. I've always liked it, but the music and the paintings are the best in my opinion.
Starship Mario was pretty awesome. Even though it's not as big as the Comet Observatory in its prequel, I thought it was a nice hub for the game. Plus, I must be one of the only people here who actually likes Lubba.