The last 6.5 phone was released in October of 2011, and that was a rugged device, so I wouldn't think you would want that one. The last commercially available released device was I believe the Motorola ES400S, which was also more of a rugged device.
Out of the most powerful, you can probably get the HTC HD2 on T-Mobile, which can even run the Windows Phone 7 or Android. That phone, however, has a capacitive screen with multitouch, which means that you cannot use a stylus (to me, was a minus).
I have the LG Fathom, which has the 1 Ghz Snapdragon processor, physical keyboard, and a resistive screen. I consider it the best phone released for Windows Mobile 6.5 due to that combination of features. You can get an unlocked version of this to run only on GSM if you want, or through Verizon. I don't think you will have much luck getting CDMA to work elsewhere, from what I've read. The somewhat equivalent of this device is the LG Expo on AT&T.
On Sprint CDMA you will find the HTC Touch Pro 2.
HTC Touch Pro 2