According to Audio Asylum, there is a Sylvania VT-231, or 6SN7GT made in 1952 that's supposed to be something special. All of the Sylvania's from prior to that (WWII era) are considered among the best (I've got several of those, but don't have a pair of the 1952's). The Ken-Rad VT-231 (6SN7GT) is another fantastic tube. It may give up a little in the midrange, but has a truly great low end. It can have either dark or clear glass, but the one that's good has staggered plates (One plate is in front of the other between the spacers, and both are mounted at a slight diagonal). I was unimpressed with the RCA red base 5692, despite their reputation. For a different type of sound the Sylvania 6SN7WGT (brown base) is an interesting tube. The W indicates a tube manufactured for low microphonics (if I understand correctly). Those from the 50's sound better than those from later years, IME. These have a slightly dry sound, but very clean and precise.
If I had to pick a favorite 6SN7, I'd give a nod to the Ken-Rads. However, my results are using them as a push-pull tube in a Berning MicroZOTL. The tubes might have a very different sound in a different amp.
