I think "October" would be my number one choice. Nice song, really been a while since they've played it, hasn't it? I think it would be really cool if they did either a piano set w/ Bono and Edge, maybe throw in "The Hands That Built America" on it. Another idea would be to have a medley, although I've never heard them do one ever, but they've had so many hits that couldn't possibly get played on every tour, but it's up to them.
Occasionally playing "Gloria" and "The Unforgettable Fire" again wouldn't hurt either. I'm happy with pretty much everything they want to do, but it didn't hurt to throw in some surprises on their last tour like "The Fly" and "Stay (Faraway, So Close!", both of them were remade very well.