I guess I never really answered this one.
It depends on if i'm trail riding or racing.
Trail riding I usually listen to older country:
Hal Ketcham, Dwight Yoakam, Garth Brooks, Hank Jr., Johnny Paycheck, Ricky Skaggs, David Ball, Kathy Mattea, Sawyer Brown, Alabama, Nitty Gritty Dirt Band, Asleep at the Wheel, Confederate Railroad, stuff like that.
When i'm going to be racing and going all out I'll listen to the heavy stuff:
Scar Symmetry, Norma Jean, From Autumn to Ashes, 40 Below Summer, Pantera, Sevendust, Shattersphere, Silent Civilian, Clutch, Ill Nino, Soilwork, Volbeat, and old Killswitch and old Atreyu (the good stuff, before these guys sold out to MTV)
I have a very broad taste in music, as those lists probably demonstrate. I'm mostly a country boy but I can't seem to get enough metal in my diet.