What is the most effective captcha / antibot protection in your oppinion?

Example captcha types:
- image (can be learned if its static)
- custom question (can be breaken if bot is customised for particular site)
- text puzzle / teaser

Captcha should be:
- as much random as possible while dont leaving chance for bot to learn..
- solved within seconds by human visitor

Ideas to improve captcha:
- implement javascript