There's plenty of good servos out there. Spend as much as you can afford - being cheap with a servo will only give you headaches and force you to replace a broken servo.
For highest speed and holding power, go with a digital coreless servo (pricey, but worth the money IMHO). Airtronics, Futaba, and JR all make very nice servos.
My personal favorite is the HiTec HSC-5996TG
. It not only performs great, but has a great price/performance ratio.