I've tried most the major brands and the SED "Winged C" are defintiely my favorites.
Here's my experiences (and knowledge of others):
Sovtek EL34WXT: These are by far much better than any of their previous offerings... they fit the same overall description as their older ones but they are much smoother. They hold their lows very well, and are voiced closer to a 6550 than probably any other EL34s. Their highs however are a bit dull.
JJ EL34 / EL34L: These are the tubes that give EL34s a bad name in terms of sounding horribly thin. They have good highs, but hold no lows or low mids. The EL34Ls hold their lows a little better.
Electro Harmonix EL34s: I've never tried these, but I believe they are the lowest noise of any of the current offerings. Typically EH tubes are a bit of an upgrade over Sovteks.
SED "Winged C" EL34s: Definitely the best quality in build terms of all the new EL34s in the sense that they will last you the longest. They don't hold their lows like the Sovteks, but still do quite well. They are a mid-heavy EL34, like the EL34s of old.
Svetlana EL34s: The cheaper versions of the SED "Winged C"... I've played plenty of amps with them (new in Marshalls) but I have never tried them in a properly biased amp so I can't really tell you much about them. They are supposed to sound like the SEDs but with less low end, and may very well suffer from not holding lows like the JJs when biased to run at the correct hotness.
Shuguang EL34-B (Ruby): Never tried these either, but I've talked to someone who used these and SED "Winged Cs" and in comparison he said these things are much more neutrally voiced, most notibly they have less midrange. I would think from that description, the heavy EL34 player might like these...
New Mullard EL34s: The new Mullard EL34s are fairly similiar to the SED "Winged C" EL34s with a little less midrange growl and the high end doesn't have the sam clearness to it.
Ei (E.I.) 6CA7/EL34: Never used these. Ei tubes typically sound good but tend to not take heat and be noisy... I've read some reviews that said the opposite, but I just read watford valves reviews which strongly confirmed this.
I can recommend SED "Winged C" EL34s as a good all around EL34 from experience, as long as you like the heavy midrange.
Edit: Do not get the Ei EL34s. I've read some reviews that make them look very intriguing, but I just read the watford valves review and they said they took little time to show signs of arcing and customers have also had problems with them.
