Very interesting topic, I have always wanted to see an ML-5, but what I would really like to see is the one that was marketed as a "consumer" deck. In the 80's ML marketed the deck along with several, I don't know the number, two track copies of masters he had licensed. They, I guess, were similar to the records that were produced, but were instead direct copies from the masters. The cost was 20 to 25K, and I don't know how many 2 track tapes came with that. Those tapes are what I would really like to see.
In terms of audiophile, I guess it is how you define it. To me "audiophile" denotes consumer use, albeit, high-end and even behind on the reach of most. Pro grear to me is not audiophile. The pro great should have the best specs, but it was not designed for consumer use. In my mind you have to distinguish between the two. If you mean audiophile in the sense of top of the line consumer decks then it would be the Revox consumer decks from Studer/Revox, Tandberg and Technics RS series. These decks were what Nakimichi was to the cassette deck (Tandberg also made audiophile cassette decks). I think those would be considered by most to be the high end of consumer decks. The Revox's were mostly low speed decks 3.75 and 7.5 although you could get high speed 7.5/15 ips. The Technics and Tandberg (depending on model) had all three speeds. Tandberg had special equalization with a seperate bias head that made it quite good in terms of home recording.
The problem with "audiophile" in reel to reel was the quality of prerecorded material. In the 50's you had 2 track but that switched to 4 track by the very early 60's. This was possible because of significant improvment in head design (head gap) and the audiotape itself. Prerecorded tapes were initially very high quality, 7.5 ips, and made at 4x speed. Meaning the submaster tape would be spun (on Ampex reproduction equipment) at 30ips and the consumer reels were spun at the same speed. Over time this eventually went to 8x, submaster spining at 60 ips, and 16x when the tapes were made in the 3.75 format. By the end only record club outlets (Columbia, the notorious 1R1 cat. no.'s, were making tapes). So while you had audiophile quality playback equipment you had very little in the way of audiophile source material.
If you are talking about state of the art in recording, pro gear, there has been some answers already which is what one would expect, Ampex vs. Studer. It is going to come down to one of those two.
Travis