Leonard H. McCoy

Leonard H. McCoy, or Doctor McCoy as he was affectionately known, never ever said "Damn it, Jim." Except that one time. But nobody ever said "Beam me up, Scotty." Now that I'm sure of. Yeah. Damn it.
In 2265, Doctor McCoy tried to sell phasers to the natives of Capella IV; the Capellans passed in favor of their traditonal weapons, the kligat. McCoy, in a later briefing, stated that these weapons were as effective as phasers. Given that kligats were boomerang-like cutting instruments and not phased energy weapons that could kill thousands with a single charge, it would appear that as a gun-runner, McCoy made a good physician. (TOS: "She Wore a Yellow Ribbon")
McCoy was a bit of an experimenter, continually treating aliens with all manner of medications, stimulants, and surgical procedures without ever having encountered their species, much less reading up on their anatomy. (TOS: "Please Don't Eat the Children", "Nocturne in Grey, No. 2", "Mudd For Sale").
As a matter of fact, McCoy performed surgery on one of the Federation's leading lights, Ambassador Sarek of Vulcan, but only after having his family sign a waiver acknowledging that he was completely unfamiliar with Vulcan anatomy. This is especially disturbing when one considers that Vulcans were among the founders of the Federation and had been serving along with humans before Starfleet was even UESPA.
Eventually, McCoy's malpractice (and alcoholism), caught up with him. Convicted by a Klingon court of flagrant medical blunders and operating while intoxicated, the good doctor was sentenced to life on Rura Penthe. Star Trek 6: Klingons get MADD.