ask how many budget u have before u ask which servo u should choose
if u have enough money,ofcox go for a better servo like BLS451/551 or something like that
if u haven't much money,but want a servo with same preformence, go for SAVOX,they have some high preformence but cheaper servo
but the quality of SAVOX may not be as same as BLS451/551
also, Sanwa's SRG-BR is good
but the price is much higher than BLS451/551(this one cost around 800 HKD)