Thread:Project:Support desk/What's the most effective CAPTCHA system?

From Extension:ConfirmEdit, which of the CAPTCHA systems is the most effective? I thought reCAPTCHA would be awesome, since it's developed by Google, but it turned out that spammers were able to bypass it as if it wasn't there. I'm not sure if the reCAPTCHA issue is an inherent vulnerability of reCAPTCHA, or if it's just the implementation on MediaWiki that has security issues which allow malicious users to bypass it. Either way, what CAPTCHA solution would be best at preventing spam?