It must really depend on what you're looking for/doing. My favorite is http://getskeleton.com/. I'm a noob, but I like that I can understand and completely customize the 400 lines of code (https://github.com/dhg/Skeleton/blob/master/css/skeleton.css) easily.

I, personally, like foundation. Mostly because it's more barebones than Bootstrap, it's fairly easy to customise and it uses SCSS which I like better than Less.

On the other hand, it sometimes feels like its development has stalled, while Bootstrap future looks bright https://twitter.com/mdo/status/591364406816079873?lang=en