I'm on AT&T but I do know of others on VM and ST. Cricket isn't offered in my area.
VM is pretty terrible with respect to data speeds. I think Sprint in general, which is what VM uses as a network, is just really terrible with it comes to data on the iPhone. So I'd say pass on that front.
ST is probably a safe bet for you. You'll pay more than VM but you'll be running on AT&T's superior network. I would just warn you though that these past few weeks there has been a lot of talk over at the Howardforums regarding poor data performance with respect to iPhone's on ST. In fact there have been some posts on this forum too recently about it. So proceed with caution.