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.