Most effective form of CAPTCHA?

Question

Of all the forms of CAPTCHA available, which one is the \"least crackable\" while remaining fairly human readable?

Answer 1

I believe that CAPTCHA is dying. If someone really wants to break it, it will be broken. I read (somewhere, don't remember where) about a site that gave you free porn in exchange for answering CAPTCHAs to they can be rendered obsolete by bots. So, why bother?