Most transparent amp you've heard?
That's easy. The most transparent amps I've heard are from Atma-Sphere, whether the M-60, MA-1 or MA-2.
What does 'transparent' mean - or what do I mean when I use that term? In a somewhat serious way, I mean the absence of the sort of distortion caused by output transformers between the gain stage and speakers. More generally I'd say, more transparency means less distortion. A transparent amp will lend itself to neutrality, but I don't think 'neutral' fully sums up 'transparent', unless you simply consider the terms as tautological.
I think of 'neutrality' almost exclusively in terms of tone character. Apart from tonal neutrality, phase coherency is an important aspect of transparency. The inductive and capacitive character of wire can introduce a sort of temporal distortion or smearing that greatly influences transparency. (...and a transformer happens to be one of the places where you find a lot of wire.) I don't think of hysteresis loss as increasing distortion as much as reducing signal, which is more a reduction of information than a distortion. But semantic entanglements approach.
Discussions of amp transparency probably should not look solely at the amp itself, but need to consider the amp/speaker interface and amp/speaker interaction. A slightly different question might be: what is the most transparent amp/speaker combo you've heard?
Tim