For retro computers, I would have to say that the 'best' CP/M machines are probably the Z80-based Altos units. Fantastic hardware; I had an ACS8000-series with twin 8 inch drives that as far as I know is still running for the person who got it from me; codeman just sold a 580-10 on eBay that boots up to three-user MP/M from hard disk (I'm glad he got a fairly reasonable price for it, too).
The TRS-80 Model II would qualify if only counting the hardware capabilities, but as I recall the major CP/Ms for it weren't exceptionally reliable. And the 'cost-reduced' Model 12 has some odd issues to keep it out of the running.
If you're interested in the most floppy formats supported, then the Montezuma Micro CP/M for the M4 is nice. The Model 4's performance is a bit less than what the Model II could deliver, though, and there is the bit with the lack of some keys on the M4 (hey, I am an M4 junkie, but I vastly prefer LS-DOS to CP/M for my retrocomputing! And now that I'm used to having some of those characters on my keyboard the lack of them on the M4 is noticeable.).
Now, if you want the retro CP/M feel with modern-retro hardware, you really should look into building an N8VEM Z80 system. If emulated hardware is enough, codeman's z-80 sim is quite nice, on pretty good hardware (I just bought a Duinomite from codeman through eBay.... just wish the TRS-80 sim code was public

.....).