Second favorite act behind Tomlin
Oh, my catalog is full and pretty varied but when I'm listening to Christian music it seems like it cycles more in the realm of:
Switchfoot
Jeremy Camp
Sent By Ravens
Relient K
Skillet
Sanctus Real
David Crowder Band
Underoath
The Classic Crime
Flyleaf
This Beautiful Republic
But for softer styles, I really enjoy Phil Wickham and Shawn McDonald a lot.
What classifies something as Christian music
Good question. I don't know how they categorize particular artists.
For example, Switchfoot doesn't promote themselves as "Christian" artists, even though they are Christians. Casting Crowns, on the other hand, uses their music as a way to try to reach people about Christ.
Really good question.