I was very fortunate to have spent some time with Rick Johnson is 1988, what a great racer.
Certainly he is on the list... although to be honest I have a hard time picking only three!!
In no particular order -
Bob Hannah - Brilliant marketer, probably copied Ali to get the folks going. Always said what he felt, even if it would piss folks off. Very fast racer, not the best riding style but determined to win...
Danny Chandler - Such determination. He was almost unbeatable for a time... but his biggest asset was that he never gave up. There is a photo of him on the internet going race speed with his bars bent like a pretzel.. I can't imagine any rider of today doing that (maybe RC but he's not racing anymore)
Special mention.
Ricky Carmichael - This is a guy with some serious heart... he re-invented himself after Bubba came on the scene, almost re-working his entire riding style to compete and beat him... plus he's a big two-stroke fan...
John Dowd - This guy's racing career spans more years that many of the racers competing today. His third place overall at Southwick this summer cements him as one of the very best..... plus he races a cool 250 two stroke whenever he can.