As the others have stated, if you want a quality product with great support, Futaba, KO and Sanwa hands down!

Yes there are other manufacturers, as noted. I run Power HD and also Protek a lot too. But in my race cars / buggies as I run a Futaba Radio, its only Futaba Servos for me.

The best thing about the big name brands is the fact that more than likely your servo may see much more duty in more chassis as you go. Also parts for these are a bit easier to come by as well.