4. Commander Barry Garner, Battlestar Galactica
Another example of a promotee, not worthy of the promotion. Garner is an engineer and had no experience in leading a team, let alone a ship. So, to prove himself, as soon as he takes over the Pegasus, he imposes too much discipline, starts to micromanage and terrorizes his subordinates, especially Starbuck. There are many incidences where he faltered, like when he sends the Pegasus on a scouting mission instead of some raptors, was a clear disobedience of orders. Then, when the Pegasus is under fire, he goes to fix the FTL drive on his own, deserting the CIC. A close runner up is Lee Adama, who delayed the events to the extent that when Pegasus comes to rescue Galactica, it is too late. But unlike Garner, Adama does destroy the Pegasus.
3. Commodore Matt Decker, Star Trek, ‘The Doomsday Machine.’
He is the sole Captain, who almost wiped out two starship crews. The first incident was when he beams down his crew on a planet which was in the path of a planet-destroying weapon. His ship was wrecked, and he comes aboard the Enterprise, and attempts to take command of it, and would have nearly got them in the way of the planet-crusher too. Ultimately, it was Decker who killed that thing but had Kirk not intervened; we would have lost the Enterprise. Not just Decker, Captain Varley of the Yomato, who got his ship infected with a computer virus by going to the Neutral Zone. And Captain Ransom of Equinox who loved torturing the aliens. And the time-travelling Captain Braxton, are also strong contenders of a mention on this list. But, we have clubbed them all under one heading.