I ride 700x60 schalbe big apples. I swap back and forth with 28c conti gatorskins every month or two - the change of pace keep me excited about getting on the bike.

If I only could have one pair of tires, It'd be a gatorskin in either a 28 or 32 for my rough urban commute. It depends on what you ride, though - you could get away with 23c race tires on some people's commutes.