Ultimate center speaker in a small HT set up?

You are going to have to do a lot more spadework

(A)There is no "best".... Full stop. Without actually auditioning any alternatives personally, all you are going to get here is a shotgun approach (instead of a rifle) of heavily biased personal value judgements pushing their faves that have about a nil chance of actually sounding good with your gear. System synergy matters.

(B) Without actually disclosing the specs of your equipment, cables, more details on the viewing environment, and especially your budget (emphasis added ) your overly broad question as currently posed is analogous to asking: "... How long is a piece of string?"

ANS: "... It depends."

Look at Legacy Audio options.

Actually Akg, I agree with you totally and thank you for that!

My room is a small one; 10 x 12. IMHO, I think small rooms are a lot easier to work with. I have a 50" Panasonic Plasma on the wall and under that a Sunuko Vent Quadrasphire AV rack (which is about 4 ft. wide). On each side of the rack, I have placed a SVS Sub Ultra 13. On top of the subs sit a granite stand on each. Under the granite stand, I have put self adhesive felt pads, so as not to mark the subs. On the subs will sit my Monitor Audio PL100 (Presently on order). Amplification is Lexicon RX7 and processor is the new Yamaha CXA-5000. Back surrounds are Anthony Gallo Strada 2. All interconnects are Cardas Clear and Golden Reference and all speaker cables, Nordost Tyr. I have presently, a Monitor Audio GXC350 as a center speaker and it is really very good. But now as I await delivery of the PL100's, I am wondering can I further improve on my center speaker? Does not 60% of the sound not emanate from the center?
Now, I am aware that the front three speakers should timbre match, but is it really so important? What if I were to insert a Vivid Audio C1 in there for example? I have heard great things about Vivid Audio and I love the look of the C1.

It surely is , all great fun!