I played on a Monette Chicago at ITG. This horn just locked right in. Whatever I did, it responded. Love at first buzz. It had alot of core, top of the staff was really centered (no need for alternate fingerings on E and Eb); I could relax and be really open all the way up to C above and even D. Playing with another player, resultant tones were really obvious and strong. (Manny and Dave who?
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I also messed around on the Eclipse, Blackburn, Stomvi, and a Schagerl rotary. The Eclipse was also easily responsive, in tune and open in feel and sound. A beautiful sounding horn that is aesthetically pleasing to the eyes as well. Again, no need for alternates on top of the staff.
Blackburn was a gem to play as well. They had a few different combinations of bell and leadpipe there, so all of them played and sounded a bit different. I did feel the need to use alternates on those, but sound quality on all horns was clear and open, with varying timbral qualities and lightness and weight with each combination.
The Stomvi I did not really enjoy as much, althoguh it still is a great horn. What I felt was not "it" for me was the lack of presence I percieved from behind the bell.
Schagerl was mostly out of curiosity, spawned by our own "Latin from Manhattan" and Mnozil Brass. I actually heard (again, from behind the bell...remember it's the worst seat in the house) alot of brilliance; it also seemed to be a bit light in sound. Maybe just the model and configuration that was there.
I also played the Yamaha Chicago. It was responsive, but not as much as the Eclipse and Monette were. The valves are quite obviously Yamaha valves...meaning they don't have the smooth buttery feel of the Getzen or Baurfiend valves on Eclipse or Monette (although...disclaimer here...the Monette WAS a Chicago, and he's making his own block now). I did need to use alternates on E and Eb.
The C in my case at the moment is a Yamaha Heavywall. Not my favorite out of all the recent horns by a longshot, but a very nice chamber/solo horn. light sound, easy to hear behind the bell, projects well, but not as responsive to timbre changes when you call on it to do so. (I've had it since early-mid '90's). It's what I have, and I make it work because I have to.
So, all that said, what's the best C? Like Nick said, it all depends on what kind of work you do the most. They all play differently, but to call one the "best" over all the others depends on alot of variables. So the real answer is...none, but all. You have to find what is best for YOU and YOUR situation and YOUR needs.