I want to buy an iPhone for my dad, which one (model) should I buy?

Answered by,

Worngal jil

I have understood your question, I would like to refer iPhone 6 or iPhone 6s / iPhone 6s , because only these model have long(Compare to other iPhone's) batter backup.

The battery life for the iPhone 5, iPhone 5c, and iPhone 5s is similar,
but is substantially improved for the iPhone 6 and iPhone 6 Plus,As the iPhone 6 and iPhone 6 Plus have physically larger housings, Apple was able to use physically larger batteries with longer run time.You can See the difference in Below Image.

I personally recommend you to go for iPhone 6s or iPhone 6s plus, its battery backup is much better then other iPhone's. Also new ios 9+ will give you an extra 1 hour battery life on Battery Saver mode.

Here are some compression between few iPhone models.(Or you can find detailed comparison at Battery life comparison: iPhone 6s and iPhone 6s Plus vs iPhone 6 vs iPhone 5s vs iPhone 5 vs iPhone 4S   this website.

Image Source:iPhone Hacks | #1 iPhone, iPad, iOS Blog

Thanks

Hope this will helpful to you to make a decision.