What is the best Reel To Reel machine ever built?

Well, while I certainly agree regarding mechanism and transport, I have good reason for recommending the C-37 especially for home use 2-track playback. Carefully serviced, their built-in amplification is about as close as you can get for the best possible in tube-based RTR playback.
Sure - there are customized ss based playback amplifiers by notable audiophile origin (Cello etc.), but they never moved the earth beneath my feet.
A top-notch C-37 may have more hiss in playback than most japanese RTRs of the 1970s/80s and certainly can't compete with Nagra either, but then you will have a very hard time getting the dimensionality, soundstage, micro-color and "live-feeling" a C-37 can provide.
After all we are talking home use playback here - not recording, not pre-mix master taping.
My vote for playback - and meant for that purpose only.
Another point - easy access (at least in Europe...) to NOS/NIB original parts. Tube-based plug-in devices can be serviced quite comfortably too.
Lastly and maybe of relevance - I am not bias on tubes.
My power amps are SEFets, battery powered - for good reason.
Cheers,
D.

Most studios in the US with functioning tape machines have newer Studers. There were still a few new ones for sale as recently as a few years ago. Sadly, most of them sit in the corner unused. There are a few engineers who still insist on them, but its getting to be a very small number-mostly engineers with private studios of their own. The pressure and speed (and budget) of the modern studio/client combo makes it a tough decision and a big commitment. Its so cool to see Stevie Wonder Innervisions liner notes and see a friend (who now works at JBL) listed as "Tape Operator".

i almost traded my Dobbins Garrard 301/Loricraft for a 220v 50hz C37 from Europe....but in the end i did not want to deal with the power converter issues. so i purchased the 'American' C37, the Ampex 350. which, like the C37 in Europe, has lots of parts available and has that tubed output. and i purchased an additional set of Ampex 351 output electronics to be able to use that too.

which is better in that light? C37 or Ampex 350/351? many different opinions on that. lots of great music was mastered on both.

i do think that neither of those machines are the 'one answer' for all tapes. and the tape transports of either are crude and abrupt compared to the A820/A80/ATR-102.

Dear Mikelavigne, yes - most great european generated music - classical as well as jazz and rock, beat etc. - was recorded on Studer - most american on Ampe(re)x.
The voltage conversation isn't all that great deal at all - this does frequently scare many audiophiles on both sides of the pond to give vintage gear from the "other side" a try.
Unless one wants to actually record / master new recordings on his own behalf, I still think it is about playback quality when talking about RTR for home use.
Cheers,
D.

having never heard a Studer C37, or an Ampex 350/351 for that matter, i can't say whether they might sound the best for playback. in fact; those i know that have the most experience say to me that those 50's and 60's machines actually make the best live recordings, but more contemporary machines sound better for playback.

i do think that quality of transport matters to the sound. it's not the only thing, but it's part of the picture.

i think we will see more custom electronics coming to market as the RTR audiophile movement matures. also; perceptions of specific output electronics is extremely context driven. i've had many tell me that the King/Cello sounds wonderful in my system, but maybe not as good in other contexts.....and i've heard the opposite too.

so there is more than one viewpoint as well as method.