Rock Band 3
Tragically, the game comes with a keyboard and not a keytar. Alas.
I'm a big fan of Ben Kuchera's writing and here's his review of the latest installment of the Rock Band series. He loves it.
Here's another review, this one from GameSpot. They love it too.
I could link to more reviews but they say the same thing. Meanwhile, Viacom is getting ready to sell off Harmonix, the company that makes Rock Band. This despite the fact that it's selling like hotcakes and everyone loves it.