If one uses 'enjoyment factor' to determine the "best pianist ever", then I'd have to vote for Victor Borge and his whacky sense of humour.
There is an urban legend that's been doing the rounds for years (but I can find nothing to substantiate it) which claims to explain Borge's switch to comedy after an incident in which Borge, while still a serious concert pianist, was on-stage with a full orchestra to perform some piece or other.
There was a difference of opinion between Borge and the conductor over the tempo, with Borge convinced that the conductor's view was too slow. When unable to convince the conductor to change tempo, Borge proceeded to play the piano part at his own tempo and, when finishing a few minutes before the orchestra, got up, bowed to the audience and marched off the stage to howls of laughter.
Be it just another urban legend or be it real, its the type of thing he would have done.
One of my favourite sketches of his is "The Mozart Opera":
Another was his conflicted duet on Liszt's Hangarian Rhapsody #2:
[YOUTUBE]-562rqZ7sKI[/YOUTUBE]
The, without the aid of any piano, there was his "Phonetic Punctuation" which still amuses:
So, Vic-baby gets my vote...
Dave