If you have solid reasons to not even consider one option, expose it here.
Feel free to propose any new candidates.
After some feedback is gathered, I'll start a poll.
Is the plan to choose the best one OR pick the best 3 of the above and have 'miners' do one of each per payout ORr are you planning on presenting 3 random captures from the 8 possible? I think the latter might be more secure. Having looked at them they all seem ok and by mixing them up, rather than using the same 3, it would give bots a significantly tougher time. And it always leaves the possibility of adding more (secure) capchas to the pool of 8.
I like the Google and the Sweetcaptcha the best
At the moment, reCaptcha, SweetCaptcha and Asirra are the strongest candidates.
Any feedback is appreciated.
Unfortunately, Asirra's server is down, and it's not the first time.
So it seems it won't be reliable to use it.