The contenders?
1. Kirk refuses to raise the shields in Star Trek II. This leads to events that cause the deaths of Spock and another crewman, the premature detonation of the Genesis torpedo, the destruction of the U.S.S. Grissom, and the death of David Marcus. Even though this action EVENTUALLY saved Earth, Kirk didn't know that when he refused to raise shields.
2. Kirk beams down to a planet BEFORE determining that it would kill his crew if he ever beamed back up. (The Omega Glory)
3. Acting Captain Riker fires ONCE on the Klingon Bird of Prey while orbiting Veridian III. This lack of retalliation results in the destruction of the Enterprise-D.
4. Captain Sisko fires special torpedoes at a Maquis planet thus leaving it uninhabitable for 50 years.
5. Q comes aboard the Enterprise. He wants to hang out with Picard for awhile. Instead of letting him stay and get bored (and maybe taking advantage of his omnipotent powers), Picard makes him angry which causes Q to send the Enterprise 2000 lightyears away to have first contact with the Borg. This will eventually lead to the destruction of 39 starships at Wolf 359 and probably more in a second attack in Earth's orbit.
6. The Dominion warns Captain Sisko that any venture into the Gamma Quadrant will be considered an act of war. Instead of heeding these warnings, Captain Sisko takes a heavily armed warship through the wormhole on several occasions.
7. Captain Janeway and the U.S.S. Voyager encounter the Barzan Wormhole. After collecting some wayward Ferengi, They stabilize the wormhole. Instead of going through the wormhole immediately, Janeway says that they are preparing to enter the wormhole in seven minutes. They're waiting for no reason at all. As a result, the Ferengi escape and they get pulled through the wormhole. Their ship destabilizes the wormhole. No going home for Voyager.
8. Captain Janeway kills Tuvix against his will. Even though he was a transporter accident and his death resulted in the resurrection of two other individuals, she still kills another person.
Feel free to add your own choices.
NOTE: Janeway's destruction of the Caretaker's array doesn't count. The Caretaker set the array to self destruct. Voyager's interference caused a battle with the Kazon. This battle is the reason that the self-destruct sequence was deactivated. By destroying the array, Janeway undid the damage that the battle had caused.
Nit. #2.
Kirk beamed on to the Exeter, not to the planet. He beamed onto the planet to keep from infecting his crew.
Okay. Replace planet with ship. It's still a lousy decision. Simple protection suits could have prevented it.
It's been a long time since I've seen that episode. My memory ain't what it used to be.
TUE
OK, I should note that the first of these decisions was bad only in hindsight. Kirk didn't raise the sheilds becuase the Reliant was SUPPOSEd to be a friendly ship. "Hey a chat with a fellow Captain!" How could he know what would happen? Frankly, in his place, I might have made the same boo-boo.
Regulations specify that if no contact has been made with a vessel, some kind of action is supposed to happen. (Saavik started to quote this kind of regulation before she was cut off by Spock)
I think Kirk's failure to raise the defensive shields is one of the absolute worst ever. They're *defensive* shields. The regulation saying "put up your shields if the other ship doesn't answer your hail" makes perfect sense. That reg was designed to protect ships and crew, and Kirk ignored it. Dumb.
But I may have to hand the worst decision to Picard, for choosing the most idiotic time to exit the Nexus in GENERATIONS.
Cisko for condoning Garak's destruction of the Romulan Senator's shuttle to get Romulus into the war. Yeah, he may have saved the Alpha Quadrant but it meant the killing of BILLIONS! We must destroy this village to save it...
Janeway for accepting command...
Yes, I still nominate Janeway's blowing up the Caretaker's array. No reason a time bomb couldn't have been set. No Kazon were near it after they destroyed the massive potato ship.
Cisko for condoning Garak's destruction of the Romulan Senator's shuttle to get Romulus into the war. Yeah, he may have saved the Alpha Quadrant but it meant the killing of BILLIONS! We must destroy this village to save it...jklien
I don't exactly follow what you mean. I don't quite see a correlation between Sisko's ability to accept Garak's actions and the "Billions" of deaths you speak of? Could you be more specific?
Janeway for accepting command...jklien
Hey, no argument there! 
Yes, bringing the Romulans into the war resulted in the deaths of many Romulans, as opposed to if they hadn't when the deaths would have only been the entire Federation, Klingon Empire, Tholian Assembly, Romulan Star Empire, etc, etc, etc....
Wait, are you seriously saying that keeping the Romulans out of the war would somehow have caused those billions to live? The war with the incredibly vicious Dominion, which was *winning* for most of the time the fighting was going on? Check your logic here, because I'm not getting it. Yes, the Alpha Quadrant suffered horribly, but it is now a *free* Alpha Quadrant.
Regarding the list above, there's one thing you didn't post that I think easily rockets to the top: Admiral Janeway's wanton disregard for anything approaching logic or sense when she goes back in time and blithely hands Captain Janeway future technology so Voyager can get home early. And Captain Janeway accepts it. Because, come *on*.
Admiral Who-ever: Captain Jones of the starship Springfield is getting promoted. You think your XO is ready for a ship?"
Captain Bob: "Admiral, Janeway will make a fine captain."
I know people don't want to remember something called Star Trek V, but...
I don't even know where to begin...
Admiral whats his face (Bob?) ordering the Enterprise to Nimbus 3.
On Voyager Janeway breaking regulations was so commonplace she almost doesn't count because the list would be too long. The guy who promoted her was guilty of following the Peter Principle.
Picard I give a pass on antagonizing "Q". Q would've found a reason for his actions anyway.
Sisko accepting Garak's murder of a Romulan Senator was an example of the end actually justifying the means. (Yes, they do sometimes. If you want to argue an absolute, PM is that-a-way.)
If you want a bad descision, Sisko starting the Dominion war by continueing to use the wormhole, has got to take the cake.
The ends may justify the means, but the means will affect the end.
-- from, The Romulan Way (I think), a Star Trek novel that includes the beginning of the Romulan empire.
I've always liked the quote.
Sisko ignoring the Prophets warning thereby allowing Jadzia to die.
In the first episode aired of the original series "Mantrap",Kirk yells at Dr.McCoy on how the sophisticated sensors of the Enterprise can locate a single human on the surface of a planet.So why didn't Kirk in "Where No Man Has Gone Before" do the same with Gary Mitchell when Mitchell escaped custody? Kirk could have located him & fired the ship's phasers from space at Mitchell.Instead,in the episode,Kirk orders everyone back to the ship while he goes after Mitchell,alone on foot.
It's possible that at the time of "Where No Man..." ship's sensors weren't quite as sophisticated as they were in "Mantrap". Also, because Gary was Kirk's friend, he may have hoped to appeal to Mitchell's humanity and bring about a resolution that didn't involve Gary Mithchell's death.
Live long and prosper.
How and who infected Kirk in Mark of Gideon? He's gonna spend the rest of his life for a girl who would have died anyways?
How did they get the plans for a starship? And there would be a riot or mass murder for that faux ship's space!
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Garek is supposed to be a tailor, not a spy...Mmmmm Odo missed something here.
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En.Kirk cost Finney his starship command chance. I bet Cogly won Finney's freedom!
Kirk was feeling guilty of losing the Farragut until his Ensign Garrovick proved the creature cant be killed by phaser fire..
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Klingons killed Kirk's son and think he was demoted for disobeying orders and he's a terrorist! in Star Trek 3.
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Spock lets wheelchaired Pike go to talos 4.How and when did he get the chance to talk to Pike on Starbase 11?
Kirk gets the fall when he lets Pike go to talos 4 too.
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Kirk disobeys the Admiral in Amok Time to save Spock's life and ends up being dead (sort of).
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Kirk lost how many redshirts? He musta not slept well when they died, especially Mr. Leslie.
How are any of those, except the last two, "Worst Captain's Decision"s?
T'pol does nothing when the NX-01 is under massive five in Azati Prime.
Picard doesn't use Hugh as walking virus in "I, Borg." Doing so may have been unethical, but it would have saved countless lives.
Who decided no shields for Memory Alpha? Dr.Double Dumb, dumb and dumber did that dangit!
were the writers asleep for that one?
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ScottN- you should question Mike's gary Mitchell post then, seriously.
terrible decision-
Cpt .Sisko got replaced by Col.Kira!
he is now on the Norpin Colony with Cpt.Scott! (no just kidding).
!, why? In Mike's Gary Mitchell post, he's questioning Kirk's decision to beam down alone, instead of zapping him with the ship's phasers. That *is* a Captain's decision. I fail to see how your post that I questioned was a "captain's decision", much less a "worst" one.