If there is one all encompassing rule in the Galactic Federation, one that EVERY officer of EVERY ship must follow to the letter, it's the prime directive.
First introduced way back in the original series, the prime directive is very simple: if a planet's dominant species has not yet achieved warp technology levels, then Federation officers are not to speak with, interact with, or share higher level tech with them if they can at all help it. It's a good rule on paper, since it basically forbids officers from landing on a random primitive planet and declaring themselves a god.
However, that doesn't stop them from doing that anyway. A lot.
The prime directive sounds great on paper, but its apparently harsh punishments for breaking it haven't stopped a LOT of characters throughout the show's history breaking this rule with reckless abandon. To the point where it's a cliche of the show that the captain will solemnly look into the middle distance and declare that they're going to break the prime directive. Which honestly loses its gravitas after a while.
To prove this, we're going to look at the 10 most notable times this happened. Whether notable for how justified, unjustified, or just plain freaking SILLY the reason is, these are the ten most memorable moments where Star Trek broke the prime directive.
13. When The Bough Breaks - Next Generation
Another one that's a bit justified, since as usual - the aliens are the ones to start the fight here. Because heaven forbid starfleet come off as anything other than f*&king saintly.
When the Enterprise crew meets a race known as the Aldeans, everything seems to be going well until their leader brings up that - oh by the way - we want some, well to be honest, ALL of your ship's kids, hope that's not an issue. Obviously it IS an issue, so the Aldeans just kinda...take them anyway.
The episode ends with Picard basically disassembling the Aldeans' entire way of life by stopping this scheme, but it's at the bottom of the list because Picard and company were LEAVING, and then the Aldeans escalated things by starting the fight. At that point, Picard has a solid defense for doing whatever he has to do in order to get them back.
The bottom portion of this list are doubtless violations of the prime directive, but as is the case with violations like these, there's a solid chance of the transgressor getting off because Picard wasn't really left with any other option.