So far I've tried the RCA/Mullard GZ-34, Tungsol 5U4gb, RCA 5V4g and Synergy Hifi 5AR4.
The GZ-34 are labeled RCA, but they were actually made by Mullard. These are rare, but not the extremely rare first version. They're labeled F-32. I've not been really taken with these in my system (First Watt F5, Druids, ASI cables, 36.5DM and Wyred DAC2). They sound kind of plain jane.
The Tungsol 5u4gb are not that expensive and sound quite different from the others. A lot more bass, but it's not as well controlled bass as the others, so it doesn't sound as tight. Not my favorite in my system, but a change of pace from the others.
I've found the Synergy HiFi to be the best for me. Full disclosure, I'm a dealer for the Synergy tubes, but I decided to use them myself, long before that. They have a very even response throughout the frequency spectrum and are the best at detail retrieval, harmonics and ambient nuance. Absolutely brilliant.
The Synergy are not that expensive at $160 per set, but for a really nice tube on a budget, the RCA 5v4g are similar in character and deliver 80% of the performance of the Synergy. These can be found for $30-40 pair and are excellent for the money.
Also, don't get caught up in whether the tubes are closely matched, as long as they're the same exact tube (meaning made by the same company, during the same period of time) and still strong, exact matching is not going to make them sound any better.