It´s impossible to know who is the best chess player in history.

stockfish9 is the best chess player.

Is it presently impossible to know the main line of the "Shortest" and "most accurate Solid winning position". If it's possible, then which best chess players played that position ?

Every chessplayer has it‘s own definition. So how to decide?

I suggest to translate the superlative „best“ with „very good“ like I used to do when translating Latin superlatives, simply a structure with „very“ or „pretty“. Then it usually makes sense. E.g. it was not the „highest house“ but rather a „very high house“ which fitted in the context.

Always those „best books, best players, best openings“: choose „pretty good“ and it‘ll be a decent question.

@oldegeezer

Do you have any examples of players where a beats b beats c beats a?

We could compare chess players' moves with moves of AlphaZero. And we can see, who has the highest amount of overlap. It was done with usual engines before, but it's of course far from accurate. But with AlphaZero (which is more human-like) we can get more accurate results. Also it depends on what does the question mean: whether we want to know the best chess player relatively to their contemporaries, or the best chess player in absolute terms. As already was said, the strength of play increases with time, and players of the current time are on the average stronger than players of the past.

@name222

really

look at tournament results
everybody loses sometime
keep looking and you will find a beats b beats c all over the place

@oldegeezer

If it's not happening consistently it's not relevant.

Probably the game of chess does not decide who is the better player of any two who are currently playing, only who won the games that were played.

Leaving aside the fact that most greats could never have met and that early greats may or may not have taken to more modern conditions, we also have the point that not all potential greats had an opportunity to achieve their potential, or greats to become greater. Examples of what I mean:
Morphy was never given a chance to become world champion.
Capablanca was never given a replay against Alekhin, while Euwe granted Alekhin one.
What if Rubinstein hadn't been so disturbed by WW1? (Capa was delighted to get a draw out of Akiba Rubinstein.)

I read this week:

The best chessplayer of all times was Erwin Wackernagl from Biebelsried, who played on the 2nd February 1922 112 blindfold games simultaneously against himself and -believe it or not- won all 112. Soon after that success he decided to rest on his laurels.

(courtesy of „Danalto“)