I mean the unlocked version. They're all the same and should work on AT&T. The one that says "T-Mobile Sim Free". Don't let AT&T get over with that bogus charge.
I was going to do this, but there is a small difference between the AT&T iPhone and the other ones; the AT&T one supports "Band 30" or something like that. Which just means that all of the phones support the current AT&T LTE bands, but the AT&T one specifically already supports the new AT&T bands coming out.