I personally think the Axial trans is one of the better options. Pretty tough stock, aftermarket to make it tougher, compact and easy to mount on a center skid.

One trans I don't see listed is the Traxxas Emaxx trans. They're a bigger trans so the truck needs to be on the larger side. But they're available as a 2 speed and should prove to be plenty tough for just about whatever you come up with.