I have dug out 'Uncle Erics' medium Mu triode tests from the mid 90s as of relevance to the DIY experrimenters here.
Top tube was a
76 Sylvania 40s, 2nd harmonic at 0.002%, Rp 37k
then
5687 RCA 50s, 2nd harmonic at 0.005%, Rp 48k
6CG7 / 6FQ7 Sylvania 60s, 2nd harmonic at 0.005%, Rp 25k
9002 Kenrad 40s, 2nd harmonic at 0.005%, Rp 48k
76 RCA 40s, 2nd harmonic at 0.005%, Rp 37k
12SX7 JAN RCA 1955, 2nd harmonic at 0.005%, Rp 37k
12SX7 JAN RCA 1955, 2nd harmonic at 0.005%, Rp 37k (another example but tested identially)
than follows 5687WB, 396A/2C51, 6C5G, 76, 6GH8A, 5814 and the first 6SN7GT
6SN7GTA Sylvania 1982 and 1978, 2nd harmonic at 0.015%, Rp 48k
and then pages more of tested valves but this is where most 6SN7GT start appearing (so quite high up the list)
Erics general conclusion were:
basically 5687 and 6CG7 are capable of low distortion but need to be selected first.
Old 6SN7GT are very consistent (span being only 0.2%). Current Russian are OK but not as good as NOS.
9002 (and Acorn 955) are good.
Antique triodes like 27, 56, 76 are excellent. [perhaps also 30, 37?]
12AU7 and 5814s are not consistent.
7199 is not standout despite being a hifi tube!
Steer clear of 5963 and 5964 as they were intended a computing switching tubes and not linear operation.
12SX7GT were used in old military radion, if you are offered some don't turn them down. [These are ones that always stood out to me on the list and why I have used them ever since]
396A,, 2C51 are under appreciated in audio circles like 9002 and 7F8 (loctal version of 396A, 2C51).
Eric was recommending tubes above 0.025% distortion which covered most 6SN7GTs.