(くろうと) Anyone who is a professional at something – more of an economic word than a compliment.
(たつじん) Someone who is at the very top of their chosen profession. The kind of talent that only one out of a thousand doctors or professors has.
(めいじん) Basically the same as 達人、 but you can use 命じん for your friend who knows everything about golf or gundams.   $$$