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

@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. 

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.