For something like a personal recommendation system, machine learning type of stuff on a website, what language would be best?
All of the points that Bill Karwin have brought up are entirely valid: You should really operate in what you are most comfortable with.
If that isn't a factor, I would personally suggest Ruby. It's an incredibly powerful language that draws a lot of commonalities with Lisp and is probably just as good, if not better, for AI programming for all the same reasons Lisp was: it's an extremely dynamic, self-modifiable language. As an added benefit Ruby has a really nice standard library including some great libs like the distributed computed library "dRuby".
Interested in Collective Programming for the web -- Ruby or Python or PHP?