who makes a decent center channel speaker?

I second Kal - same brand and even better if same type - not always practical though.

"I would stick with the same brand as your left and right channels" (Mlawitm)

"Not only with the same brand but as similar to the L/R speakers as possible. I prefer and use three identical speakers across the front" (Kr4)

"I second Kal - same brand and even better if same type" (Shadorne)

I wouldn't even think twice about using a different center than your mains! You'd be better off either with no center at all, or using some other combo of L/C/R speakers as your multi-channel setup, rather than mixing and matching speakers across your front, yes! (it's basically like haveing a different left and right speaker in a stereo setup - you wouldn't do it).
I basically disagree with the line of thinking of getting a "good center" to go with your mains. This is a SURE way to end up with an incoherent, totally unbelievable soundstage, different sounding pans across the front 3 speakers - drawing you out of the illusion of one soundfield! And yes, every professional audio reviewer will agree here. But, you make your choices, and live with your results, either way.
So what are your left and right speakers, and what's your setup like now?

I agree with the first reply, Mribob. Arial CC5 is one best Center. But if you are asking for a DECENT one, hummm, it'd be lots of them on the menu.... Good luck finding.

I wouldn't think twice about using a different centre channel than what is used for the left and right channels in a home theatre application. However, it is critical that the centre be tonally matched. The centre speaker reproduces information that is in phase, i.e. mono. Usually this is voices that are in front of you on the screen. The centre speaker can be optimized for the reproduction of midrange, which is the frequencies where the voices are located. When voices are off-centre, the information is out of phase and in stereo. The left and right speakers will take of care of it, but they must be tonally matched so it doesn't sound different from voices reproduced in the front. Using identical speakers all the way across obviates the tonality problem. And that's fine, but it fails to take advantage of the speaker specialization that the centre is designed for and which enhances the home theatre experience.

Music and home theatre are not the same. You can't automatically apply what you would do in a two channel stereo to a multichannel home theatre. A single centre speaker is not a stereo speaker, Neither is a single sub. A sub is another example of a specialized speaker that exists in home theatre that many two channel people have no use for. Same logic and principle as for a centre. You give something up, i.e. integration, in order to get something else, i.e. low frequency reproduction of effects at louder volumes. It's not needed for most music, but it enhances the home theatre experience.

On the other hand, for discrete, multichannel music (or older, alternative forms of stereo using three speakers), I would want all speakers to be identical, front, centre and rear.

Markphd: "The centre speaker can be optimized for the reproduction of midrange, which is the frequencies where the voices are located.".................."And that's fine, but it fails to take advantage of the speaker specialization that the centre is designed for and which enhances the home theatre experience."

Can you tell us what those 'specializations' are? What optimizations for voice distinguish it from proper main speakers and do not compromise frequency response?

Unfortunately, the construction of the vast majority center speakers is faulty BY INTENT as they cater to aesthetics and convenience rather than performance. This leaves the users of those companies main speakers in the lurch, without a satisfactory alternative other an using an additional main speaker.

Kal