Best CAPTCHA I ever came up with was a pattern-recognition one, where you'd have to type out the missing word. I had sentences such as "North, ____, East, West" and "____, South, East, West", etc. The bots didn't stand a chance.
....oh, did I forget to mention that it was in Hebrew, for an Israeli site?
Take this, bots! "צפון, ___, מזרח, מערב"
The second-best, suitable for a more global audience, can be
seen here. The secret was that the CAPTCHA generator script was able to use TrueType fonts, so I grabbed a bunch of very funky ones off of AcidFonts (which our very own and much-missed Setekh turned me on to years ago).