This one is tough.
If its an album artist where a person sings, then my favorite is Billy Joel. He's one of the better singer/songwriters and he's still making albums to this day, so that tells you something about his longetivity.
My specialty is soundtracks/scores and compositions. Its impossible to pick a favorite because each composer has a different style. If I'm in the mood for a certain theme, then that's what I'll choose because most every film has a major theme it is based around (i.e. the Jurassic Park theme). Most of the stuff I have is done by John Williams, James Horner, Yoko Kanno, and Joe Hisaishi.