There really isn't a lot to say that doesn't border on generalities to some extent. Hate to be another "it depends on what system and where in the system" poster, but it is true, and often expensive to determine.
If I could suggest only one thing, it would be to find a tube dealer you trust and who knows your equipment, and then TRUST them to steer you to the best tubes available.
In addition, the link provided by Clueless in this thread also points out the variations among brands and designations within brands. This is a widely read link that contains very good baseline information, but may be followed in an overly "biblical" way by some.
For instance, one might think that the pinched waist Amperex are the "holy grail" in this family of tubes, but I can tell you from experience that this is not always the case. The article does not mention that these particular tubes have a high variability in terms of microphonics and are dang hard to match.
I have found that, in general, that the Mullard tubes can usually be characterized by more "warmth" than, say, a Telefunken, but perhaps not always as extended. The very best of the Teles and Siemens are surely very articulate and extended, but sometimes lacking in the midrange, depending on production dates. I have found the Amperex, especially 60's U.S.-made to be very good "overallers" - the white label PQ's besting the earlier Bugle Boys, which are often touted as King.
It is very hard to beat the early-late 60's Teles. IMHO, even the non-gold pin 6dj8's in the line sound head and shoulders above the later 6922 and 7308's which can sound rather dry (I will stick to U.S. designations, here, instead of ECC88, etc.). For Siemens it is a similar situation. A CCa of the 80's cannot hold a candle to a 60's version, generally. BUT, you pay for the pleasure, of course. A great point that "Joe's Tube Lore" alludes to is that many of the earlier tubes can be found under other labels (i.e. HP) at reasonable prices, but ya gotta dig, or rely on your supplier.
Another thing is where in a system the tubes is used. If the placement is in a low gain situation, then the audible difference between tubes will be less noticeable that in a something like a phono stage. It seems kind of strange to me that many will argue over brand and type of tubes when they are using them in something like the output buffer amp of a CD player (a mostly no-gain, non-voltage application) Here, the tube brand is probably one of the least noticeable variables. Others will likely argue this point but, hey, my skin is thick.
So, I would say that the term "rolling" tubes was come by honestly. Often it takes several rolls (as in dice) to come up with a winner. Often the winner is not what you'd expect from looking at the "headliners" and "price lists". For instance, I just parted with a Kora Eclipse, a very fine preamplifier of which I had two other previous versions. It is a triode design based on using three 6922's. I tried various and sundry Teles, Siemens, Amperex, Mullard, and Valvo tubes - and within the brands 6dj8's, 6922's, 7308's, etc. etc. When all was said and done the very BEST sounding tube in that piece, IMHO, is La Radio Technique 7308's. Not a tube that would have appeared on anyone's wish list, not all that easy to find, but a clear winner. Go figure....then, to really give things yet another spin, think about the 6N1P's, 6FQ7's which can SOMETIMES prove to be nice replacement....
