Hitec 645MG in my opinion, I think most will reccomend the 625MG which has less torque and more speed, but if you're like me, and like to drive as fast as possible while turning, you'll want to go w/ the 645MG. Both are around $40.
when u get those, do they come with the little grommet thingys that keep the mounts from cracking?, those stupid mounts are the only reason i have to get new servos....