AVID 6x12x4 Revolution
The Revolution Series bearings are a low friction bearing with several options to help satisfy anyone's desire. What makes this bearing unique is that it has a rubber seal on one side while having a metal shield on the other. Rubber seals are great protectors against dirt but the price is that they offer a little more friction due to the tighter seal.
6x12x4 Revolution Halo bearing by Avid Racing Concepts is a popular bearing for vehicles/makes like the T-Maxx, E-Maxx, Tamiya's, and others.