T-Mobile objectively has the best network if you have good reception. They locally peer, you're getting your own IPv6 address, and their interconnections are excellent (except with AT&T as AT&T refuses to peer and they use Zayo which will take you across the US).
AT&T by far has the worst network, even if you have great service. They NAT millions of devices behind a single /64 network, you're usually routed to IXs very far away from you, and they do the most packet manipulation out of all the carriers. Verizon is better, but they can do some wacky stuff if you aren't in New England or California.
AT&T by far has the worst network, even if you have great service. They NAT millions of devices behind a single /64 network, you're usually routed to IXs very far away from you, and they do the most packet manipulation out of all the carriers. Verizon is better, but they can do some wacky stuff if you aren't in New England or California.