If you like RPGs you'll like Oblivion.
Forza 2 is fun, but frustrating to me.
A first person shooter would be COD4, 3rd person shooters like Bioshock, Fallout 3, Fable 2 and GOW2 would be good if you like those types of games. Fallout 3 roughly = GoW2.
I honestly like GoW, but Army of Two was better IMO.
Of course if you're good at video games, get them all. Then you'll become an achievement whore. In the past month I went from 5000 to 8155 just by playing different games.
As far as music games, I got started on GH but Rockband with the stratocaster guitar is so much more fun... not to mention easier. GH WT on expert tends to use A LOT of triple finger chords in the harder songs, which makes it a pain in the ass to me compared to songs like Panic Attack (on RB2) which is a lot of hammer ons.
Personally I'd go for Rockband 2 with the entire instrument set if you're going to play it a lot.