I'd pick Cancer. First reason being it's a badass song. IMO it also has a rock radio friendly sound, too. Second reason because it rocks from start to finish. The first two singles both started out slow, Cancer is hard right off the bat. That way people who aren't familiar with the whole album won't think the songs always follow the same format. Next reason (tying into the fact that it's one of the harder tracks on the album) the crowd always seems most enthusiastic when the singles are played, the casual fans know them. Take that enthusiasm, add Cancer, equals a whole bunch of people going crazy, thus adding to the whole concert experience.
So that's my pick. I think about these things way too much.
![Tongue out :P](./images/smilies/tongue.gif)