Click-captcha has tiny touchpoints, so I fail a couple times on mobile, just from that. Phpcaptcha has autocorrect enabled on the text box, so "alk" was changed to "all", causing a fail. Captcheck worked well.
I know it's not as usable for visually impaired people, but I kinda like those that ask you to drag a piece of puzzle on an image : https://github.com/ArgoZhang/SliderCaptcha