Best center I have heard is a pair of 20.1s with their tweeter strips abutting and angled sharply. ;-)
I am about to order a Salk Soundscape center channel (to use with Avalon Eidolon Vision mains). This center is $3,000, but from a manufacturer direct operation with no overhead. Avalon will release a center that has the same level drivers shortly, which will sell for 10-15K. The midrange driver is accuton and the tweeter is a ribbon. This should be an extremely fast and transparent center, and match very will with electrostats. Bit of a leap of faith of course....