A favorite throttle servo is the JR DS 8231 ... mostly because I have several of them laying around from helis. These are fairly high torque, higher than average speed coreless motor digital servos with excellent centering. They seem to last forever and have an extremely free gear train. When the power is off a throttle return spring can easily close the throttle if a servo were to fail. Again, I differ from some people and fail to see the need for metal gears in a throttle servo.