I don't know what data speeds you'll get, but T-Mobile and most any carrier EXCEPT Sprint or Verizon. (and the prepaid carriers and such that use their network, such as, I think, Virgin.)
You will get LTE speeds on any AT&T MVNO such as Net10 or Cricket. Straight Talk as well, but I'm not as sure on that one. The phone does NOT have to be unlocked to work on Cricket, I know this for s fact as that is what I'm doing