For something like a personal recommendation system, machine learning type of stuff on a website, what language would be best?
The language you know best would be best.
I mean that half-seriously.  Given the brief description of your project, there's no reason to believe any of the languages you list would be any better or worse than the others.  Those three languages are adequate for approximately similar tasks, so you should pick the one you are most comfortable with and proceed.
Any other recommendations would be advocacy for one language or the other, with no real basis on which to evaluate them.  Your description is just too vague.
Interested in Collective Programming for the web -- Ruby or Python or PHP?