Your best bet would be Straight Talk. It's $45 USD, no contract. You just put in a nano SIM and activate. Unlimited talk and text and 3 GB data. Straight Talk resell ATT bandwidth, so you've got good coverage (do some research relating to coverage in your US area.)
PS: provided your iPhone 6 is truly unlocked. Whatever you choose, don't choose Sprint. Verizon will only work if you have a VZ unlocked model, and even then VZ is persnickety.