If you understand the limitations of each model of transmission there isn't an Allison transmission that is a bad transmission.
The AT models are lighter duty and do not have the lock up feature. Downhill can be a real exciting experience if you expected the transmission to slow you down on the way down the hill. The AT will be found behind the 5.9L, T444(E), DT(A)360, and some of the other members of the DT family depending upon the HP rating.
The MT models are medium duty and will be found behind engines over 210 HP to 300 HP and feature lock up. They will hold back going downhill.
The HT models are heavy duty and will be found behind engines with lots of HP and torque and feature lock up. They will hold back going downhill as well.
Some of the MT and HT transmissions have five gears but most have four gears. If the model number has a five in the center position it is a 5-speed and if it is a four it is a 4-speed.
The newer Allison models 1000/2000/3000 feature OD gears and lock up. In bus applications the 6-gear is usually locked out so the double OD gear isn't used. They are great transmissions but do have the added problems of electronics.