Hmmmm! That's a hard one, I love so many! >.<
I guess if I was foooorced to choose... probably Orbital, I grew up listening to all their albums and I could listen to them forever, I love electronica music and theirs is more inventive and different sounding between albums...
And then either Yasunori Mitsuda or Nobuo Uematsu, who did the soundtracks to Chrono Trigger/Cross and the Final Fantasy series, respectively. So much range of music and it's all so fun to listen to, even outside of the games =D
Aaaaand maybe it's just cause I only discovered him recently, but I think Wax Tailor has an amazing set of work, and I could listen to it all day and never get bored <3
But man there's soooooo much good music to choose from, I dunno how to even pick favorites! :3