the best of the best of tube preamps.
Well clearly that has to be my preamp :)
Seriously though there are a lot of issues with what is thought to be the 'best'. Part of the problem is that some people actually *like* solid state, so it might be that you have to sort out the "best" transistor and "best" tube preamps.
Its my opinion that the preamp story is incomplete without the experience of the interconnect cables. You might say that the cables, especially the one between the preamp and the amp, is a hidden cost of any preamp.
One of the functions of the preamp is to control the interconnect cable and preamps range from having no control of the cable at all (passive volume controls) to the cable making no difference whatsoever in the sound of the system (that will be a balanced line setup, BTW).
And since there is the issue of balanced, if your amplifier does not accept a balanced input you may have to further differentiate between balanced and single-ended preamps. IMO balanced is the way to go as the system will have lower noise, less artifact from cables and power supplies, and less distortion from the preamp itself (and if you get rid of distortion, the preamp will sound smoother and more detailed at the same time).
The problem is, no audiophile has heard everything, and even if they did, it would be highly unlikely that they heard it under ideal conditions to show that piece off. Now if that could be done, maybe we *could* arrive at "best" (as the English language has the word 'best' for a reason); there probably is a 'best' in some sort of existential way, but getting agreement on that (despite its actually being the best) is unlikely regardless. So 'best' having the meaning it does, keep in mind that when manufacturers tell you that they have the best that they are most likely to be lying or uninformed- the word gets used a lot...