Depends on the workout.
If it is a 2 hour bike ride, I'll listen to Red Hot Chili Peppers, Mishka, some Saliva, and a few others with the power song being Dani California. Mainly stuff built for the long haul.
If I'm doing a circuit where intensity is key I'll go into a lot harder stuff...some Metallica, Three Days Grace, Sevendust, a little Static X, maybe some Korn, and a few others with the power song being Pain.
You need music that motivates you, but it has to motivate you in the right way for what you are doing. If I was planning on riding for 2 hours and listened to Black from Sevendust, I'd be dead after the first 20 minutes.
Best advice I can give is the tempo of the music has to match the tempo of the activity.