best preamps

Some of the preamps that have been mentioned have output impedances that are too high at some frequencies to be optimal (or even suitable) for use with some power amps. Some of the preamps that have been mentioned have input impedances that are too low to be optimal (or even suitable) for use with some source components. Some of the preamps that have been mentioned have too much gain to be optimal (or even suitable) for use with some combinations of source components, power amps that have high gain, and speakers that have high efficiency.  Some even have too little gain to be suitable for use in some systems.

An indication of what associated components the preamp would be used with would result in answers that are more focused.

Good luck. Regards,
-- Al

So let's see about 24 responses to the OP and around 27 suggestions......that seems right!

;)!

A very general question will only yield a very general response. In addition, there is no "best", only a best for you. 

MY ’Best Preamp" is the one I ended up getting for free. I ran into a used Audio Research SP-10 locally at a dealer, for $800, of which I traded useless junk to lower the price to my $600 cash. Within a few weeks after, I bought a used Audio Research SP-15 off the goN’ for $1500. So I had both at the same time. The SP-15 crushed the SP-10 sound wise. No question. (though if the SP-10 had the best tubes ever, it might have changed the balance. The SP-10 uses like 11 tubes.)
So the typical SP-10 price on Audiogon was $3K. I listed my SP-10 for $2300 (notice $1500 and $800 equal $2300) and said buy some good tubes. Amazing I posted the ad at 7AM, and in half an hour had five offers to buy. (I should have asked for more money? Anyway, the SP-10 sold. And I basically got the Sp-15 for free.
Free is good. Best deal I ever did. So that is why the Audio Research SP-15 is the BEST PREAMP in my opinion.