If battery life is your primary requirement then get an old phone. I get around 5 days out of my Samsung Tocco Ultra; its an adequate enough phone, camera,video, internet etc.
On the Android front my Galaxy S2 with the default battery does well to get 24 hours with heavy usage. (Wifi,GPS,lots of apps, background syncing). Think being careful I can stretch it to twice that (maybe more?).
Can definitely recommend you look into 3rd party extended batteries. Picked one up for a tenner on ebay inc a new back to fit it in and now get around 50% more life.