My all-time favorite is the Nokia 3310. Probably hard to find these days. The games were fun, the construction sturdy, the interface simple, the lines clean, the buttons large enough, the ringtone loud enough (and customizable!), and best of all, if you nicked up the outside, you could buy a new one without having to replace the whole phone.
Ultimately, the best phone is the one that is what you need it to be.