Park CC2. Very quick and easy to use. I can check chains on six bikes in my garage in one minute - I can do it without taking the bikes off the hanging hooks, I can do it in sub-optimal light, I can do it without putting on my reading glasses, I can take multiple readings quickly at different sections of chain without moving the crank. The tool errs a little on the high side, so if you really want to make sure you don't change your chain before it's reached the maximum recommended elongation, you can switch to a ruler after you get past 1% on the chain checker.

Chain tool threads are a common source of bickering in here - people who disagree with me are wrong.