To what extent does Tom Buchanan value power over others, and how does this manifest itself throughout The Great Gatsby?

Tom values power over others to a great extent. At the beginning of the novel, he is unable to fully control his wife. She pokes fun at him and his ideas, calls him a "brute" even though he hates that word, is angry with him for his infidelity, and feels disillusioned about their life together. This may be part of the reason Tom has so many affairs. He likely enjoys his ability to control Myrtle Wilson: when he orders her to "Get on the next train," she does it. The one time Myrtle disobeys him, saying Daisy's name over and over, "Tom broke her nose with his open hand." It seems Tom greatly enjoys the level of control he can typically exercise over his mistress; when he cannot control her, he lashes out. 

Tom also sadistically enjoys exercising control over Myrtle's husband, George, a mechanic who desperately wants to buy Tom's older car so that he can sell it and make some money. When Wilson asks about it, and Tom replies that he's having his man work on it a bit, Wilson says, "Works pretty slow, don't he?" because Tom has delayed this transaction for some time. Tom responds that his man does not work slowly and threatens to sell the car elsewhere. Tom enjoys dangling the car in front of Wilson, watching the poor man who lives a sad, spiritless life in the valley of the ashes and whose wife is Tom's mistress, backpedal and defer to what Tom says. 

After Tom realizes Daisy loves Gatsby and George tells him that he learned Myrtle has been having an affair, Nick sees that Tom is quite concerned: "His wife and his mistress, until an hour ago secure and inviolate, were slipping precipitately from his control." Tom hates being out of control, and so he reveals the illegal means by which Gatsby has made his money (bootlegging), partially bringing Daisy back into his control. When Nick next sees the couple, sitting at their table, Tom's "hand had fallen upon and covered [Daisy's] own," so it seems he regains some semblance of control over his wife, having lost it utterly over his mistress (who is now dead). 

It seems that, in most of Tom's personal relationships, he aims to retain the upper hand.  He orders Nick around, he orders Myrtle around, he manipulates George to feel more powerful, and he even successfully brings Daisy into his control by convincing her to abandon her lover.