Depends on your ability and the hill. I generally switch between an 11-23 and 12-27. The 11-23 really is only for flat and fast events, but sometimes I get lazy and don't switch it off for normal rides. Most of the stuff I ride is rolling hills or extended 4-5% grade type of stuff, nothing that pitches up to 10% for long periods of time. The 39/23 works well enough for that, though I end up mashing a bit more than I'd ideally like to do.

But again, depends on your fitness and the terrain.