I agree with Ralph that implementation outweighs the actual tube. That said, I'm quite pleased with my current NOS Mullard EL34's and slapped some into my guitar amp too. I've always wanted to hear some GM20's.
What is your favorite tube type?
With so many tubes that do different things and have different sound profiles, what is your favorite? EL34s wonderful PRaT, KT88/6550s smooth and dimensional sound, lush mids of a 300B, detail of a 2A3 or the huge soundstage of an 845? I am curious which is your favorite. What do you like about it?