The best center speaker is the exact same speaker as the two front speakers. If that is not an option, then the center channel speaker that matches the front speakers is the next best choice. A center channel from a different manufacturer is the most problematic option, as the sound will change dramatically as it pans from one side of the image to the other.
What brand and model of speaker are you using as your front pair?
What brand and model of speaker are you using as your front pair?
