I tend to use either JJ or TAD (Tube Amp Doctor) valves when I replace mine - though not because I've done much research or sonic comparisons, rather that they are decent valves for the money and seem to perform well enough for me. Very few valves are made anywhere as well as they used to be back in the '50s and '60s, so NOS valves can sometimes be worth the extra if it isn't a ridiculous premium being asked. Plus the more your amp's worth, the more it's worth spending a bit more on the things that make the sound.

Even within a brand, there are normally several options for better quality valves at a slight price increase. The 'gold pin' versions of valves are often worth paying a few extra dollars for.