A "stankovite" is an overachiever. More specifically, a stankovite is someone who exceeds work quotas or targets set by management by such a great margin as to affect the setting of those quotas. A stankovite is somebody who works so much harder than everyone else that he makes everyone else look really bad for doing all that was expected of them.
It is a term of derision and insult applied by fellow workers to someone who makes their lives miserabel by validating management's suspicions that they could all be producing at much higher levels if only they tried.
The term originates from an actual man, whose first name I can't remember, named Stankovitch. He was a coal miner in the Soviet Union in the 30s and 40s who was marketed as a "hero worker" by the Soviet authorities for exceeding his cola mining quotas by vast amounts through unceasing work, a model of the "new socialist man's" self-effacing dedication to his society.
It was later revealed that Stankovitch's production numbers were inflated by authorities who created artificial work situations that allowed him to produce far more than was possible, no matter how hard one worked, under normal work conditions.