12AX7s are one of the few tubes that are really good in current production. If you have a chunk of change then you can opt for the NOS or OS or used older production tubes.
One tube you should try regardless of your taste is a 5751.
Because of high demand and small production runs during the hey day of tube Hi Fi real New Old Stock of any make is very rare and will be pricy. If you can tolerate used then the best are mid fifties 3 mica Black Plates in this orders Sylvania, RCA, or five star GE . Check the Agon accesories for sale tubes and you can get a feel for prices.
If you are not rich then try the JAN 1970-1980s vintage 5751s by Phillips with Sylvania or GE or just by Phillips. These are quite good and you can get really new unused for less than 30 bucks each more or less I should say. They seem to be asking more for them of late now that everyone is broke.
Current production The Tung Sol 12AX7 Gold Pin is a good tube for something smooth. If you like it crystalline and tipped up with treble then consider the Sino or Shuagang 12AX7c or 9th ed. is best but rare. The 12AX7B or 8th is easier to get. It will cost you at most 10 bucks from a fair tube seller. Many may think I am joking but that C version is really a good tube.
For older tubes you will probably like most of the Phillips factory Tubes. The British Mullard not reissue especially early 60s or older long plates are excellent but again not cheap.
A very good imitation of the Mullard not fake from the 70s are BEL look to tubemonger for those. Its an Indian electronics manufacturer using Phillips tooling when they made the small tubes . They don't now. Another Phillips tooled tube is labeled often as a Mullard but made in Japan by Toshiba and Matsushita are quite good actually.
One of the best is Amperex Holland 12AX7s but a pair in good shape will cost a lot I would guess only guess that you would have to pay $200/PR that's not for the very top tubes either.
I like the Tesla not JJ which are pretty good and inexpensive current prod themselves (ECC803s). the Tesla is from the 70s I think and get the Crossed Swords version . There are a huge number of phoney tubes around so be careful even Teslas which are not highly sought after have been copied.
There is an old article called Joes Tube Lore on Audio Asylum FAQ section that is way out of date regarding prices calling the best 5751s cheap but runs the gamut. I didn't even touch upon the German tubes with Telefunken being considered by many the best but certainly not for warmth. Good luck
