Depending on your budget but to be honest i recommend any of the following both good reliable servos does exactly what it says on the tin
The XP pulls and turns a tank as used to run these in my truggy and cannot fualt it for power and steering response i have now got the xp in my buggy and the steering rate has had to be turned down alot as to fast;
If you wish to spend a bit more then you could go for this baby;
http://www.rcdisco.com/part-info.php?partNo=KO30100
Very good servo and built for quality but a tad expensive but depends on what each driver wants from a servo