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

If you're just looking for a captcha to prevent spammers from bombing your blog, the best option is something simple but unique. For example, ask to write the word "Cat" into a box. The advantage of this is that no targeted captcha-breaker was developed for this solution, and your small blog isn't important enough for someone to actually develop one. I've used such a captcha on my blog with some success for a couple of years now.