A question best considered in a variety of ways.
Best build quality?
Most technologically advanced?
Best tape path handling?
So the best sounding reel to reel tape recorders, many would suggest would be one of the top Studer machines which is an easy assumption based on build quality and application.
But certain hard-core tape recorder enthusiasts will maintain that some machines made in the US in the late 50s, such as Presto, Crown and a variety of other companies are the best sounding tape recorders ever made. Some of the new generation tape recorders that have been manufactured or (refurbished from older models) in the last five or so years might be at the top of the list as well. Generally speaking Japanese tape recorders should not be included in the top 10 list unless you are talking about reliability.
What is the most reliable reel to reel tape recorder ever built? To get to this answer one would get more specific and suggest it would need to meet two criteria
It never or rarely needed to be serviced
Has record and playback heads that are more durable than regular permalloy.
There are quite a few Japanese machines that would be at the top of this list