This depends on whether you're talking indoor sport or outdoor alpine, but layers are always a good idea so you can add or strip layers as you get too warm or too hot. This is far more of an issue for alpine climbing where you may be starting and stopping, waiting to belay someone else and then racing to try to get up to the top of a pitch. It's easier to answer for sport climbing: close-fitting comfortable breathable clothes that won't get stuck in your belay device or snagged on anything while you climb; hair fasteners for women to keep hair back and out of the way; and no rings or jewelry so they won't get stuck in cracks, or you don't loose them or so they won't get all scuffed up from the hard rock surfaces. Leave them at home.
