I think that Slim is probably the most impressive character in the text. Consider the way that Steinbeck describes him when he first walks in as an example of how impressive Slim is:
A tall man stood in the doorway. He held a crushed Stetson hat under his arm while he combed his long, black, damp hair straight back. Like the others he wore blue jeans and a short denim jacket. When he had finished combing his hair he moved into the room, and he moved with a majesty achieved only by royalty and master craftsmen. He was a jerkline skinner, the prince of the ranch, capable of driving ten, sixteen, even twenty mules with a single line to the leaders. He was capable of killing a fly on the wheeler’s butt with a bull whip without touching the mule. There was a gravity in his manner and a quiet so profound that all talk stopped when he spoke. His authority was so great that his word was taken on any subject, be it politics or love. This was Slim, the jerkline skinner. His hatchet face was ageless. He might have been thirty-five or fifty. His ear heard more than was said to him, and his slow speech had overtones not of thought, but of understanding beyond thought. His hands, large and lean, were as delicate in their action as those of a temple dancer.
This description features much within it that makes Slim impressive. The fact that he is so skilled and commands so much respect is impressive, but his lack of ego is even more stunning. Whereas so many, such as Carlson and Curley, seek to make something more of themselves, Slim is presented in a very humble manner. The worn nature of his hat reflects this humility as he is a worker. Yet, one cannot turn from his skill and craftsmanship. Steinbeck calls him "master" and this might reflect how impressive he really is. His skill with a whip to be precise and accurate is also very impressive. His ability to listen to others in a manner that mkes them feel validated while he can speak of nearly any topic such as "politics or love" is another impressive element. Finally, the last detail of hands like a temple dancer convey a sense of skill and grace to his being.
In a novel where so many characters have something disappointing about them, Slim stands alone. He defends Lennie when he is in trouble, serves as a confidant to George, and asserts a role of leadership amongst the other ranch hands. He has the attention of Curley's wife and is the only man that Crooks respects. Slim steadies George at the end of the novel. Slim is impressive, from the opening description to the last page of Steinbeck's work.