keycaptcha... will never use anything else now. Pay for the upgrade so the tiles are puzzle pieces, makes it simple for your users on desktop and mobile devices... especially mobile devices, touch and slide to put the puzzle pieces in the right place. Very intuitive system they have, and nothing has busted it to date.
I personally like recaptcha. It has never failed me. I know that it can not be that effective toward spam but I personally like it. However I am looking at the new keycaptcha, it looks pretty awesome and seems to keep away the spam.