What carrier would be best for any iPhone newer than the iPhone 4?

I'm about to do some research but I'm hoping someone here already knows this. I want the cheapest unlimited talk, text, and data for two iPhones. Any information I failed to provide I will be glad to add.

The best carriers are at&t, verizon, tmobile… Sprint and straight talk have better prices and deals

It depends where you live.
In my city, Sprint has horrid, joke like service, but maybe in your city it's perfect.

If you want to save money, you can try a carrier like PagePlus, Cricket, etc.
They basically "piggy back" from a major carrier. PagePlus uses Verizon's cell towers, Cricket uses AT&T's, Boost uses Sprint's, etc.
Now, if you buy the phone from an MVNO (like PagePlus, etc.), you'll need to pay full price for it. Could be $500+ per phone.

Look at each carrier's website and look for a service map. It will give you a pretty good idea as to what kind of reception you'll get in your area.