man, it really varies...
i like my old iPod classic just for the big "clicky" buttons (no touch screen)
Sometimes intense stretches of twisty road classical music or opera keeps my brain engaged (Tchaikovsky, Mozart, Carmen)
Hot sunny slabbing days call for Jimmy Buffet or something equally whimsical...
But, I also find Godsmack, Zeppelin, U2, Jack White, Mumford and Sons
Other times - just straight up old country (especially down south) George Jones, Twitty, Ol' Hank, Kristofferson, Alabama, George Strait