That's a very objective question. If you have previous Ruby/Rails experience, CakePHP may be the best bet. If you want a complete decoupled framework where you have complete freedom to make the architecture as you see fit, then perhaps Zend Framework is best for you.
There are of course others, most notably. Here are the ones I find most notable:
Zend Framework
CakePHP
Symfony
Kohana
CodeIgniter
Those are the top ones on my list, although I've only played around with a few but really work in CakePHP and Zend Framework.