You didn't say what kind of riding you're doing, so I'm going to assume XC.

I use these:
http://www.mtbr.com/cat/drivetrain/p...93_135crx.aspx

I know they can be a pain in the arse to flip, but when I'm riding more aggressively, I like the security of being able to have one of my feet free so if I think I'll need to based on the trail, I can dab if I need to (just in case I can't unclip fast enough to do it). Sometimes if I'm not sure of a new trail, I'll flip 'em and stay unclipped until I am more familiar with it. I do prefer to clip in for drops or jumps so my feet don't fly off the pedals (!), but for going uphill, clipping is a necessity.