I don't know from best, but when I think back on our years of Bee coverage, my favorite speller never to win was Tia Thomas. I loved that she had no poker face -- when the word came out, her eyes told you that that word had no chance against her. And when, once a year, later and later in prime time, she got the one word that she hadn't already mastered, you could see her wobble, flinching as if insulted by a close friend, and it was heartbreaking.
The level of institutional Bee knowledge here today is awesome. Anybody else have a favorite who never wore the crown?