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

It depends on the system (as a whole) and your personal preference.

I have various Mullard/Blackburn), Amperex/Holland, later 2003-2004 (when they momentarily remedied the reliably issues) Sovtek as well as a few manufactured in Japan.

They all sounded good (to me) depending upon the gear/system I used them in and my favorite varied with each/different setup.

Reliability seems to be an issue with contemporary products, but I've mainly read about this in guitar audio forums.

Never owned a DH/SET amp that was tube rectified and have used them in vintage Pilot and Dynaco PP amps as well as Fender Princeton/Deluxe and Super Reverb guitar amps.

As an example, when I ran early production (fat sounding) Mullard EL84's in the the Pilot (232/240) amps I preferred the slightly lighter sound of the Sovtek and Japanese tubes.

If I used old production EI EL84's (lighter sounding than the Mullard's) I preferred the slightly richer sound of he Mullard/Amperex 5AR4's.

In my Dynaco (ST-70/MkIV) gear, which always ran Mullard XF1/XF2 EL34's, I preferred the slightly more detailed Amperex over the Mullard version in the ST-70's and the Mullard in the MkIV's.

I've forgotten the model/version codes on the old production Amperex/Mullard 5AR4's (recall 31/32 maybe), but can tell the earlier versions by looking at the plate construction (welded VS crimped tab construction).

This said both types sounded good/close.

Many say the metal base Amperex version is the holy grail, but I've only heard them in unfamiliar (to me) systems both HiFi and guitar/amp based - so no opinion on that.

Sorry if this does not answer your question.

DeKay

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