Super Mario Galaxy 2 is what I'm playing now. I'm not too far into the game, but I'm loving it so far. The challenge level is at just about the perfect level for someone who has played the first game. Yoshi appears very early on and can be used on Starship Mario anytime. The game is great about introducing moves/power-ups/items and giving the player a fun and practical use for them right away. Production values are on a par with the first game, which is just fine.
The story is the only drawback so far. The opening scene was so brief that they may as well have just had a text screen that told you to rescue the princess from Bowser's clutches. I don't play Mario games for plot, but I would rather have nothing than a scene that doesn't add something new or make sense on its own.