What is the best workout song?

That highly depends on your tastes. Some people like a loud, upbeat song to get their adrenaline going; others like a calm, slow song to set a calm mood.

Personally, I like to listen to any of Eiffel 65's songs when working out; Roots Manuva's "Chin High" and Slick Rick's "Children's Story" work in a pinch.