Best 300B tube for Cayin 300B amp?

Occanica,
You don`t mention a price range,300bs can cost anywhere from 150-2000 USD.A competitor to the Royal Princess in it`s price range is the Takatsuki TA 300b(Japanese).
EML XLS and mesh 300b are very well regarded.

I`ve been using the Shuguang Black Treasure for two years and it sounds fabulous and has been absolutely trouble free. For 600 USD they`re worth every penny.
That new Takatsuki is very intriguing.

Ocenaica, the fact of the matter is that there is no set answer. A while back the choice was clear, if you had the scratch - Western Electric. Now there are several contenders at the top - those mentioned above and a couple of others - as well as some good second tier ( and much more affordable) options. In my opinion, if you can resist thinking of it as a lot of money for tubes, but instead view the expense only as "bang for the buck" improvement of your system, compared to the expenses of all your other components and ancillary equipment, the best are well worth it. Which of the best is the best for you? That will be determined by your personal taste and system synergy. I’m very happy with the Royal Princesses.

I would like to stay in the $600 per pair price range or less. Thanks for all of the insight, Mark

PS. System is as follows:
Nottingham spacedeck TT with SME 309 arm and Zyx Airy 3 s cart.
JLTI phono pre amp, Cayin 300B integrated amp, NOS tubes as rectifiers and for the pre section.
Omega Max Hemp speakers interconnects are crystal cable....

There is a pair of EML's on sale right now, so, maybe you can give them a try, if you like of course.

They are not my.