Dave Sinclair of Caravan
Robert Wyatt
Guy Evans of VDGG
And if charm and originality isn't enough, the virtuoso madman of all time was Jan Hammer during his Mahavishnu days. Check out Birds of Fire.
Some of the suggestions on here are pathetic. I enjoy a bit of duran but to cite the keyboardist as anything special let alone the best is like saying that Peter Taylor was the best English football player ever. FFS.
Dave Stewart of Hatfield & The North, Khan, National Health etc . because I love that Canterbury fuzz organ sound. I'll also throw in Dave Greenslade and Joe Zawinul.
Technically the best, or best at playing what a song required? Eddie Jobson, Joe Zawinul or Rick Wakeman in the first category, Nicky Hopkins, Thomas Dolby, Bernie Worrell or Rick Wakeman in the second.