If you think about the best IDE you will probably think about eclipse with the PDT-Plugin, Netbeans or Kommodo. None of them has special support for the Symfony-Framework. As I wrote in an earlier post I’m using eclipse for my daily work. With some enhancements it is a pretty good IDE for Symfony-Development. Netbeans catched up with the last releases an there is a special Symfony-Support planned in one of the future releases.