I use ZF the most.
Though I just started using
Symfony2 (still in early development) for a new project. Watch the
talk by Fabien for a good overview. It's
nothing like symfony1 (which is A Good Thing imo). It's a bit difficult using a framework with hardly no documentation and that changes all the time, but it's been a pleasure. Symfony2+Doctrine2+Twig=good times