I like 4-6 players, with 5 being the ideal.
Main reason is covering the party roles. With 4 players you might just manage to cover all bases, but you'll most likely end up without a rogue or healer (or even *gasp* mage). But having five or six characters generally gives all of your players the chance to choose their own character without concern for what the party needs. I'm DMing 6 players right now and though it's harder to create chalenging encounters (normal CR's don't really go anymore), it also creates a more fun and diverse party.