I have hardly heard all the contenders, and no solid state amp in my own system for quite a while, but, I've heard a few in friends' system and at shows, etc. I generally find Ayre electronics to be consistently nice sounding in a wide range of systems, so I would give them a nod. I've heard BEL amps sound good in a couple of systems too.
A friend has a First Watt amp (Nelson Pass design) in a high efficiency system that sounds very good in that system. I believe the model he has is an F2 amp. It is, however, an amp that is not as universally usable as other solid state amps -- it is somewhat low in power and is better suited to speakers which have a high impedance.
The better solid state amps that I've heard at shows are not American in origin. In particular, I thought that the Swiss Soulution amps sound good, as well as another Swiss amp, the Dartzeel amp. It is unfortunate that both are quite expensive. The German Burmester gear also sounds pretty good to me.
