Hey there! Well as the title says I'd like to know what game is the best?

Because I only have Halo 3 and Gears of War 2, so I'd like a music game. And I decided to buy a Guitar Hero to begin with, but then I looked at some forums and quite a lot of people liked Rock Band better. I'm from Denmark and I have 800 dkk wich would be about 140 dollars.

So what do you think is the best game?