If you must have… a 4G phone - Sprint
I would advise against 4G for another generation or two, as every 4G phone owner I've spoken with has complained about the short battery life (as in three or four hours max). That's why Apple is skipping 4G for its next phone.
That said, if you must have a 4G phone, you have a few choices. We're skipping T-Mobile because it's not 4G, period. HSPA+ is quite fast but 4G is either LTE or WiMAX, not a 3G on steroids. Likewise, we're skipping AT&T because it's only just beginning the rollout process.
Verizon has four Android phones, ranging from $249 to $199 with a two-year contract. Sprint, though, has five phones, with the cheapest being the Nexus S 4G from Google. This is a phone with a beautiful AMOLED screen, Near Field Communications and total voice command support, and the price has just been cut to $99.
Plus, Sprint has another sexy 4G phone coming, the Motorola Photon 4G. So the early launch of its 4G network has helped Sprint get a leg up on Verizon. Let's hope it can keep something of a lead, if only for its own good.