I've got to think before I answer this. If I don't think I would say KenRad 6C5 in adapters from
@Deyan. In the same adapters you can use 6J5. These are 'metal tubes' and much cheaper than the all glass versions (6C5GT and 6J5GT, often with and added /G as in 6J5GT/G). C5/J5 are glass tubes inside the metal. I haven't yet tried the Russian equivalent 6C2C/6S2S that is said to be very good and much cheaper. These are all input tubes (in the front).
I have tried very many combinations indeed. It's more a matter of different good than better. Sometimes you find the best combination and this impression will last until you listen to different music.
If I had to use just one combination with native transformer (no external PSU) it would be EL42 with Melz 6N12S. Unfortunately these Melz have become very expensive and always were in very short supply. Another good alternative as output tubes is either 5687 or E182CC. These use the same adapter. I bought mine from xulingmrs before I became aware of
@Deyan.
Other people have other favourites and rightly so