The very best center speaker I've ever owned was a KEF Reference Model 100 that I got for just over $100, shipped. It blended perfectly in the 5.1 "frankensystem" I had, consisting of that center, a pair of Hafler 300 floorstanding speakers, and a pair of KEF iQ10 bookshelf speakers as surrounds.
Those Reference Model 100's can still be found on eBay for about the same money, and they are really VERY good.