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 wonder if a CAPTCHA mechanism that uses collage made of pictures and asks human to type what he sees in the collage image will be much more crack-proof than the text and number image one. Imagine that the mechanism stitches pictures of cat, cup and car into a collage image and expects human visitor to tick (checkboxes) cat, cup, and car. How long do you think will hackers and crackers will come up with an algorithm to crack the mechanism (i.e. extract image elements from the collage and recognize the object depicted by each picture) ...

Answer 2

I use uniqpin.com - it's easy to use and not annoying for users. So, bots can recognise a text, but can't recognize a image.

Answer 3

If you're a small enough site, no one would bother.

If you're still looking for a CAPTCHA, I like tEABAG_3D by the OCR Research Team. It's complicated to break and uses your 3D vision. Plus, it being developed by people who break CAPTCHAs for fun.