In my very heavy DMing since 1977, one player stands out in my mind: Bill
Bill played with me from 1984 till about 2001 and he never learned the rules. He never knew what dice to roll. He always forgot his adds and minus' in combat(did I mention, in RL he was an accountant!). Luckily, he mostly played fighters and I didn't have the nightmare of him running a spellcaster. Why was he my fav of all time? Roleplaying.
He never let the rules interfer w his RPing, He was consistantly in character and RPed his ass off. We played a year long solo game of Star Trek from security cadet to Klingon admiral (He was Star Fleet in classic trek era, GURPS before there were rules) He played an albino female fighter at the end and as the dice always hated him , he had very low HP. Real life drew him away from gaming (he became a real life hero, volunteer fireman) I always hope the winds of chaos will bring him back some day.