The best phone on T-Mobile right now is either the HTC Sensation 4G or T-Mobile G2x. My wife has the Sensation and I have the G2x and they are both excellent phones.
The Hercules (T-Mobile's SGSII variant) bests them both in terms of specs and is due to be released in late October. I ruled it out because it's too big for my tastes at 4.5" but for some that huge size is a selling point.
Wow this is old. Galaxy IIS and the HTC Amaze now but also you can use the Samsung Galaxy Nexus (unlocked) which offers a bigger screen than the SII but also gives up a few specs so depends on your needs.