3GS hands down, why? it was my first iPhone for one, and to this day I still think it was the best form factor...that is until the 5c.
I have the 5s and love it, but HATE the form factor, so I can't wait until next years 6c which should have the 5s internals and hopefully be in all black!
See I liked the 3G form factor, I really wish they would have made a 5C with the 5S stuff (Touch ID, etc)