You want the best battery life...just ROOT phone and install any one of these 2 roms that I have personally used Catyrom / ZEUS. Here are some additional things you can do to greatly improve battery life...
- display - put brightness all the way down OR at the very least keep "automatic brightness"
- Turn off sync of apps that you don't use - these apps sync data continuously after designated time
- I turned off Facebook, Twitter , weather app notifications (previously they used to sync every hour = kill battery life!)...consider doing that
- Install some free widget that are 'toggle' for common functions such as bluetooth, wifi, data, 3g etc. on your homescreen... *tip* when u go to sleep turn off wifi + data ... ur sleep remember so ur likely not using ur phone ... plus its a healthy habit anyway (this will greatly imrprove battery life)
At the end of the day keep in mind that a smartphone offers alot more functionality...GPS, internet, texting, calling etc. .. and its not likely gonna last u 3 days w/ normal use.
Having said that with CATYROM 2.1 (ICS theme edition) I have been getting 1 day 5 hrs w/ minimal use which means no texting, calls ~ 1 hr, screen on/browsing/ 4 hrs, gps 10 minutes
Good luck