Most transparent amp you've heard?

What do you mean by "transparent"? It's a very inconsistently used term among audiophiles, in my experience.

FWIW, I've have come to use the word transparency with a fairly literal definition: the ability to see what comes before it. The more transparent a component is, the more I will be able to observe differences as changes are made ahead of it in the system. It's not a perfect definition for all sorts of reasons, but it I think it holds up pretty well. This definition is likely to correlate with neutrality, but it doesn't have to. Seems to me that neutrality stands on its own pretty well as an audiophile term, as do Pabul57's "low noise, speed, space between instruments, and a sense of immediacy."

Is the Audiophile Dictionary JGH's thing?

I'm being fussy about this because transparency is used so often by audiophiles and I have no idea what they mean by it.

I'm in complete agreement with Mike. I've owned Placette products and they definitely qualify. No tube preamp I've ever owned qualifies, though there may be some out there, but that's not to say they don't have a lot of other things to recommend them.