NT 3.50, service pack 3.
Ed Curry claimed he could have had 3.51 certified fast. NT 4.0, no way.
Eventually, many service packs later (6?), NT 4.0 managed to pass a UK test which Microsoft claimed was "equivalent" to the US test. I've not seen proof of that equivalence, nor have I heard of any later version of Windows passing such a security audit.
Cheers,
Ben