On technical ability, Keith Emerson and Rick Wakeman have got to be up there. Ray Manzarek, for his use of his feet to play the basslines, got to be a good shout too. Just for touch, sensitivity and colour, Lloyd Cole (assuming Lloyd Cole was the pianist with Lloyd Cole and the Commotions) is a...
He was a consummate - albeit sometimes very difficult to listen to - musician. He was a guitarist - as am I - but I've no doubt the clever sod could probably play keyboards - or any other damned instrument he chose - better than I can play guitar!
Oh, and about this 'best keyboard player'...
There's a lot more George Martin keyboards than Billy Preston on the Beatles' output, a lot more. Seem to remember Mal Evans was the roadie, and Nicky Hopkins regularly toured with the Rolling Stones, as well as recording with Quicksilver Messenger Service..