GRW bearings are the best you can get.  Period.  If you want top quality, then these are the bearings you want.  Used by top motor builders around the world for Eurosport and G7 motors, these bearings are thicker than cheaper bearings, and are designed to handle the higher load generated by high-end motors.  These bearings measure 2 X 5 X 2.3mm and are shielded, non-flanged.