Rockband 2 is the best bet if you only want to buy one music playing game.. Plus you can always expand the library.

User Rating: 9 | Rock Band 2 X360
Out of the numerous music games out there it makes one think of, " Which one can I buy that generally has all the songs so I don't have to buy a new game every other month." Well your answer would be Rockband 2. This game has a huge orginal Library to start with ( Of coarse you need to unlock songs as you go along. ) And once your done playing and conquering in your domain, you can always check out the Rockband Network and download songs that you like individually, instead of buying a new game. Plus, you can import some songs from the other Rockband into Rockband 2. Rockband 2 stands up to Guitar Hero on so many levels.
One for example can be that there are around almost 10 guitar heros and only 2 Rockbands, Well 3 Rockbands if your counting the Beatles. Also Rockband 2 has a pretty durable drumset Along with a bigger guitar. This is a very sturdy and well thought out game that puts the gamers and there wallets in mind more than the Guitar Heros. This is the Best choice for a Music game, Plus at Wal-Mart you can find it on sale.