Its a question which is impossible to answer and in the same line as "what is your favorite food". My favorite food is very much depending on my "taste of the day". Today it was beans and patatoes, but tomorrow I might prefer a steak.
But nevertheless I gave the question a good thought. To start, I limited my choice to freeware. And immediately came to the conclusion that are far too many models to choose from. For me the highest quality model isn't automatically the best model. It should give me a good feeling as well.
I recently flew Rick Piper's FS9 Provost for a while and I still like it as much as the first time I flew it. And today we have loads of scenery to fit the model. I have the same feeling with Dave Garwood's Hunters. And there are many more. These models are super nice and give me a great feeling.
But I have to choose one, as there can of course only be one "Greatest Of All Times". And when I have to limit myself to one, it must be a completely different one.
Stuart Green's Fokker D.VII was its time very much ahead. It is nearly 20 years old and when released it had animations unseen before. The detail level was icredible. And even according todays standards, it is still an amazing model. And flying it still gives me that special feeling.
So today my choice is this one. Tomorrow I might choose Milton Shupe's Beech D18 or his Douglas A20G. Or the BF110 nightfighter by the GroundCrew, or Lagaffe's abd Nieuport 17, or one of Manuele Villa's incredible models, or.............
As said by trying to answer this question, you can only make a fool of yourself