For my father, it would be this: The Asus ZenFone 2. Very good specs as his current phone is running android 2.4 and cannot browse the web anymore and has put off replacing it for about 3 years. This phone is very up to date and should be for some time:
http://www.amazon.com/ZenFone-Laser-Unlocked-Smartphone-Storage/dp/B015AH8TQE