What is the best sounding GZ34 / 5AR4 rectifier tube you've ever heard?

I think a lot depends on the circuit it is in. Yes, the old Mullards are known for long life and are a good choice. I have had the fat base (first bakelite after metal) and a metal base, which is really a Philips Miniwatt. The metal base has phenomenal bass- and at least in my application-- power supply of phono stage--is dramatically different than the later tubes in sound. It is also hard to find, pricey and may or may not be "the best" in your application. I found bliss with a GEC u52--which is not a direct equivalent. I ran a brown base with the cup style bottom for 5 years-- I’m sure there is still life in the tube, but I tracked down another--purportedly never used, black base- which as far as I can tell (open to more info) may be an industrial tube. It sounds pretty close to the brown base. Difference between that and the more pedestrian NOS Mullards- much airer highs. Difference between the GEC and the metal bass Mullard- completely different tonal balance. The Mullard metal base has profound lower registers but is not as open sounding on the higher frequencies. This may be the result, not only of the circuit, but also how my system is voiced, i.e., the combination of individual components, how they play together in my room.

I often refer people to DubStep Girl’s Massive 5AR4 Shoot-Out. You can find it just by pasting those words in a browser. She gives a run down of a lot of tubes with brief listening notes on their character. Some of the tubes she mentions-- certain WE-- are pretty much unobtanium.

I have two favorites which depend on how you want to shape the tone of your system, and I have compared these to at least a dozen other variants.

The Philips 5R4GYS from Holland is a wonderful one. It brings thicker harmonics and bloom and wide soundstage. It is easy to fall in love with especially for vocals and horns, but it is just a bit more rolled off / less prominent on the very top.

My other favorite is the EML 5U4G mesh. This one has just a little less bloom, perhaps slightly less output overall compared to the Philips, but it has a tighter bass and more articulate upper treble. Much of the rest is very similar in tone to the Philips. 

So one is bigger, bolder, with great bloom, perhaps just more euphonic overall, the other is a bit less bloomy but with more information on the frequency extremes. 

I’ve had great experience with metal base gz34’s.  I’ve used one in a Bob Latino’s sl70 power amp, Modwright pre and currently using one in my Cary SLP05.  In all three cases the metal base GZ34 made the most significant positive improvement to SQ I’ve experienced.  

Bliss:

There are some major spec differences between 5ar4 and 5r4gy(s).

Aside from a considerably higher voltage drop the 5r4 also has a faster warm up time.

Not familiar with 5u4g, but once looked into the other and decided not to go there with my amps.

DeKay

@dekay Good point. While most units that utilize GZ34 and 5AR4 can also accept other variants like 5R4, 5U4G and 274B, not all do. You have to make sure they can handle the differences. That said, I’ve owned about a dozen units from different manufacturers that used these variants, and only one of them, a custom 45 Korneff clone made by Oliver Sayes, would not accept a 5R4. That was an incredible amplifier nonetheless.