The stock servo in the RTRs are junk to begin with. There are tons of better servos out there and not many that are worse

It depends on how much you want to spend as which to get. The faster and more torque a servo has, the more it cost. I have a Solar servo, I won't buy another one. Savox is pretty good but you need a high voltage receiver (not spectrum) for it to work properly.

Futaba is one of my favorites. A little pricey but worth the $$ IMHO.