As NogDog said, it depends on what you want your framework to do for you
In my opinion, After trial of some frameworks , Codeignitor(CI) may be the simplest one .. Easy to learn ,easy to migrate to it .. And powerful introducing too many helping tools and functionalities to allow you organize and code easily , smoothly and effectively with little secure code !
Some other frameworks are more powerful such as YII allowing a great GUI (called GII) for easily creating your well written PHP classes , then you have to just edit , But its more sofosticated and not as easy as CI.
Others like Zend are in the middle of them ...
My advice , If you still want to break the world of frameworks , then start with Codeignitor .. You will love it and may work with it for years ..