It's an impossible subjective question to answer.
I'm classically trained but admire Tatum above all other pianists, so would add my 2 cents here.
For jazz pianists he took the genre technically to a new level, where, to be honest, few could even try to follow - he has been described as the greatest jazz soloist.
For classical maestros, his lightning fast improvisations were daunting. They could play as fast or possibly faster once something was learned, but Tatum could improvise at breakneck speed and yet maintain a beautiful touch and musicality. His improvised rhythm and harmony continues to wow me after 25 years of listening.
That aside, I honour him for something else.
We have recordings of him playing wonderfully on terrible pianos.
To quote: "Tatum doesn't seem bothered by the condition of the instruments he has to play...he seems to adapt himself so well to their various shortcomings that one gets the feeling he enjoyed the challenge - circumventing dead keys or adapting himself to different kinds of out-of-tuneness..."
Here's a link to him playing an upright that "was not a full-keyboard instrument". We'll never know what the action felt like, but we can hear the wear & tear and guess.
Also it's unlike his commercial studio or public recordings of standards, which he could reproduce almost mechanically, almost note for note. Here he's having fun relaxing after-hours with a good friend on whiskbrooms, and produces a mini-masterpiece of medium swing jazz.
The tune is called 'Fine and Dandy' & is little played now (though you can see it being murdered on youtube by a couple of people):
https://www.savefile.com/files/1549980(skip the advert if it pops up and scroll down to download)
Regards