Mitchell and Sheppard were mere copies IMO.
Mitchell might have started as a copy, not just of O'Neill but even John Crichton was in there too. But by the end of season 10, the writers or Browder seemed to be getting a better handle on the character, unfortunately the show ended. Cameron Mitchell vastly improved the moment someone realized to stop treating him like a comedic character and start treating him as a serious one. Although he still got his share of sarcastic one-liners that are the due of a Stargate lead, the character seemed a lot more serious than O'Neill and Sheppard. One of the few reasons I think SG-1 should have gotten another year is just so we could have gotten to further flesh out and develop Mitchell into something other than a clone of the previous Stargate leads and Browder's other famous character.
He was a career military guy who lost a son to gun violence. He was not supposed to be making jokes in almost every scene.
In the early seasons O'Neill really was a rather serious minded person who simply had a severe problem containing his sarcasm, which according to former USAF Chief of Staff General Michael Ryan (who had a cameo in SG-1's Prodigy) is very common among USAF Colonels. It was later years when RDA grew less interested in the role that he turned into a goof-ball, though that had an entertaining side to it, IMO.
Anyway, my ranking of these characters:
Jack O'Neill, is simply the best. An entertaining guy to watch on weekly adventures with and an all around compelling character.
John Sheppard I like they intentionally made him sort of "anti-establishment" or at least as much of one as a military officer can be. He had definite skill and talent, but took a laid back approach to military lifestyle that would have ended his career had he not had the Ancient gene. Basically, an everyman thrust into a position of authority he didn't really want, but pulls it off all the same.
Cameron Mitchell, as I said earlier started off too much of a clone of Sheppard, O'Neill and Crichton. While the character vastly improved once everyone decided on a different direction to take the character, we didn't get enough of the improved Mitchell to warrant a higher ranking.
Samantha Carter, though there is much to like about the character, her time in the Colonel ranks was a bit of a disservice to her. When she got promoted to Lt. Colonel things started out okay with her rightfully taking command of the SG-1 team. Then she gets shoved back to second as Mitchell takes command and when the character gets promoted to "full-bird" Colonel and transferred to Atlantis, she's basically reduced to being a substitute Dr. Weir.
Everett Young had potential and I do have a bit of a soft spot for people burnt out at their job. Unfortunately, inconsistent writing throughout SGU and ham-fisted attempts at making the character so disinterested in his job he became incompetent really were a detriment,
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