What is the best sounding vinyl?

I don't know of any Reference Recording that would be considered extremely good in terms of musical content although they are very good sonically.  For Jazz, I can recommend Harry James and His Big Band "King James Version."  

I would also recommend M&K Realtime direct to disc recordings (not recorded to tape, the live feed went directly to the cutting head making the record lacquer).  They did some decent jazz recordings in terms of music, and of course, spectacular sound.  Look for covers of Ellington classics called "For Duke," an Earl Hines recording called "Fatha," Freddie Hubbards "Back to Birdland," and Ed Graham's "Hot Stix."

As others have mentioned, there are many great jazz recordings that have been reissued by the likes of Analogue Productions and other companies that sound fantastic.  There are WAY too many good reissues to get into here.  But, I will mention that a good reissue of Sonny Rollins' "Saxophone Colossus" will blow your mind in terms of how great good mono can sound; this is great music--sample "St. Thomas" off that album, a truly great track.  For a great stereo Sonny Rollins album, look for reissues of "Way Out West" (I believe my copy is from Analogue Productions).

Many older original vinyl releases of jazz have not been matched by reissues.  This might be the case of the original tapes deteriorating a bit.  While Blue Note reissues can be really good, many originals sound better.  Unfortunately, Blue Note issued so few copies that originals are extremely pricey.  You can do better price wise getting original Columbia Records jazz recordings because they had larger pressing runs.  I demonstrate for friends how shockingly good original Columbia stereo recordings from the late 1950's sound by playing Ellington's "Blues in Orbit" and Brubeck's "Time Out."  Both have good reissues, including good digital versions, but the original "six-eye" Columbia's are even better.

One more killer good sounding album is that of the Japanese group "The Yamamoto Trio--"Midnight Sugar" on Three Blind Mice (TBM) label.  I know there have been many reissues; I have the original release but that has become quite expensive.

The Mary Stallings "Fine and Mellow," mentioned above, is a terrific sounding album from a small label, Clarity,  that specialized in ultra high quality recordings.  I have that album from Clarity that came with two discs, each having only one side pressed on them.  Another Clarity record I use to demonstrate great sound is Claudia Gomez "Salamandra." 

Clarity also made really good CDs.  One of the CD's containing Stravinsky's "Rite of Spring" has a warning about its extreme dynamic range--if you set your volume level too high for the extremely soft beginning, you could damage your speakers when the loudest passages kick in.  

Another vote for Sheffield Lab and Ian Janis Breaking silence. If I may add Kraftwerk The Man Machine German OG 1978. A 'killer', like Michael 45 would say. Mofi are hit and miss for me. Analogue productions yield better results generally.

Another vote for the ’Breaking Silence’ lp. I got it when it came out and it is quite masterfully recorded.

Otherwise, Reference Recordings label, recorded by Keith O. Johnson. Quite a number of albums on that label are superb sonically, but, as someone else noted, more for sound that any particular genre, although Dr. Johnson’s Astounding Sound Show is pretty cool, what with one cut being African drummers entering the rear of the room, moving forward and around the microphone and then exiting at the back of the room again.

And, of course, Mercury Living Presence Records (Hi Fi A La Espagnola is a killer album). 

(Harry) Belafonte at Carnegie Hall (originally issued on RCA), which was reissued some time ago by Classic Records (the original is still every bit as good). Ella Fitzgerald, Clap Hands, Here Comes Charlie (if you can find it anywhere. Verve Records. Good Luck with that!) And, the awesome, impossible-to-find Casino Royale album from 1967, with Dusty Springfields penultimate version of ’The Look of Love.’ Even harder to find than Ella. Was an audiophile favorite of good ol’ HP (The guru of The Absolute Sound).

The best sounding vinyl, to me, is what I have in my collection. Carefully curated and collected over the years. Some albums are not the best fidelity-wise, some are muted, some are new vinyl, some are 60+ year old pressing that have been listened to for decades with scuffs, wear, and skips. Yet all are the best sounding to me - even considering that I might have duplicate copies on CD or higher-res digital versions.