There's no CAPTCHA that can't be cracked. The best are based on non-visual alternatives, such as sound recognition (poor for accessibility) or sentence structure analysis (difficult due to ambiguity), as these technologies are generally not as developed as image processing is. Still, even these technologies will, in time, be cracked.
Rather than finding a good-quality CAPTCHA online, which will likely be cracked within the next six months if it's popular, you can farily easily build your own. If you have a niche website, where you can be certain that everyone will have a fairly high level of knowledge on a specific subject (e.g. yarn on a knitting forum), you can make a long list of questions which will be almost impossible to break, unless the spam bot make really, really wants to get into your knitting forum
CAPTCHA needs to be easy for humans but hard for machines, with the rise in stuff like voice and OCR recognition technologies, machines can interpret represented information in much the same way a human can (visually or aurally), therefore the only way to approach the system is cognitively (which is why they make captcha's complex such as solving a math equasion, stating the contents of an image or recogntising warped text). Unfortunately anything which involves cognitive recognition (above the norm) immediately discriminates against the disabled causing a paradox of compatibility... whatever doesn't work for robots, doesnt work for humans.