I usually work out to entire albums or dvds..and it depends on my mood, how much time I have, how much energy I want to expend...etc.
For high energy day workouts the best albums are: POP, AB, Boy and UF. I also like the Rattle & Hum cd, but I can never get past Hawkmoon

I just play it over and over and over. So, if I had to pick the very best workout song...Hawkmoon.
I usually workout very, very early in the day, outside under the stars, while it's still dark. The Joshua Tree is very moving at that time of day...
But, honestly, I will workout to any and every U2 song. I have tried other music...nothing else works. I have tried without music...forget that!