It has traditionally been impossible to get an unlimited data plan on prepaid service, but AT&T has just rolled out that very thing. GoPhone customers who want to use a lot of data can do so for a mere $60 per month. If that sounds like a good deal, get ready for the catch. This is basically the throttled Choice "unlimited" plan AT&T offers as a budget option for post-paid lines.
The unlimited GoPhone plan has the standard 22GB soft cap. If you use more than that, AT&T may slow your speeds during times of heavy use. On the plus side, no overages. The throttling includes 1.5Mbps for video and 3Mbps for everything else. That's like a fast 3G connection, but it'll be delivered over LTE. The data can be used in Mexico or Canada as well, though.
Technically, this plan costs $65 per month, but there's a $5 discount for auto-pay. If the limits placed on this plan seem unacceptable, you can still get a more standard 6GB capped GoPhone plan for $40 per month (with auto-pay credit).