You can't go wrong with this standard Kyser capo.  I use it every week at least once and never have a problem.  It's easy to put on one-handed.  I attache it to the end of my guitar when not in use.  It works beautifully.  I have bought many knock-offs over the years - especially for students who are just learning and don't want to spend much money on stuff.  I always come back to the Kyser.