If it's a pre-BB10 phone, you want to make sure it has the UMTS 1700 band.
If it's a BB10 phone, you want to make sure it has: UMTS 1700 (aka AWS, or UMTS band IV,) UMTS 1900 (aka PCS, or UMTS band II,) LTE band 4, LTE band 2.
Here are the models of BB10 phones that work well on TMO:
Z10: STL100-3
Q10: SQN100-5
Q5: SQR100-1
Z30: STA100-5
Passport: SQW100-1 (this is the only model that's available at this moment)
Classic: SQC100-4