I have phases of music I go through. I'll be stuck on one group or genre for a while and switch to another. Its a very random thing. My preferred genres are jazz of any type, classical of about any era (using classical to describe orchestral music is a very broad term), movie scores, classic rock, some alternative rock (I don't like it too hard, the likes of Alice in Chains and Metallica is as hard as I go), some R&B, a bit of Motown, good covers of songs, and some concert band literature. I also love to transpose music so I can play it on my saxophone.
I'd be hard pressed to pick a specific group, composer, or artist as my favorite, but my favorite piece of music of all time is Spencer Davis Group's Gimme Some Lovin' (the original version, though, I also like the Blues Brothers' cover).