The sound of a tube is heavily dependent upon the design of the circuit it is used in. What sounds best in one design may not be the best choice in another application.
As Onhwy61 said, it depends on your circuit. If your amp will run OK with a 5U4G that opens the door to some nice black plate RCA's or perhaps some Mullard GZ32's which can be very nice. The Mullard GZ37 High Wycomb are wonderful in some applications. If you're stuck with 5U4GB, I think the Tung-Sols are about the best sounding in the various amps where I've used them. Currently, I'm using some Sylvania 5931's in my Pilot SA-360 amp and a Mullard GZ32 in my Modwright Transporter. But I didn't like either of them the other way around, that is, with the Sylvania's in the Transporter or the Mullards in the Pilot (altho they *were* pretty nice there).
In other words, try some varieties assuming they will meet the electrical requirements of your amp. Check with the maker, or check specs on Duncan Amps site.
Hope this helps!
I'm surprised no one has mentioned the EML (Emission Labs) 5U4g mesh. In my Modwright Transporter, and in my Woo headphone amp that rectifier was wonderful, and topped my list when rolling in both applications. YMMV according to the application/circuit that you are using it in though, but the EML is most definitely a contender for top billing.
I agree with the NOS recommendations above. It's worth trying a few. The coke bottle RCA black plate and GE 5-rib skinny 5u4g have been exceptional in my Melody Pure Black 101 pre, in Quad amps and in my guitar amps where appropriate.
But I wanted to mention one current production inexpensive tube. Tubestore.com has a private-labeled and highly-tested version of the Shuguang 274B. It is available as a Shuguang tube, too, I think, but with Tubestore you can be sure. It is a heavy glass, large plate 274B with a lighter 5ar4-like filament current draw, and has much harder vacuum than the basic Shuguang tube. These are under $30 each, and have sounded very fine in anything I've tried them in. They feel robust, haven't flashbulbed like so many other current production cheap rectifiers. Absolute longevity is TBD. It's borderline for some amps, like a Mesa Mark V, that are aggressive with voltage, but it seems copasetic with most hifi amps stocked with 5u4 rectifiers.