Beast and the Harlot by Avenged Sevenfold, definitely my fave.
YYZ by Rush is pretty good too, or hell, even John the Fisherman by Primus or Misirlou.
They're all a lot of fun.
Here's a list of the 360 exclusive songs.
Makes me wish some of these songs would be released in some sort of booster disc for the PS2 version as well. And if its true that they plan to release all of the Guitar Hero 1 songs then there's no reason to have the PS2 version at all. Hell, when Resident Evil 5 comes out I'll have to break down and buy a 360 and have to end up with another copy of Guitar Hero as well.
You should enjoy it.