Sophomore year of college, someone kept prying open the washer/dryers in the laundry room, throwing the clothes on the floor, then using the machine to do their own laundry.
One day as I was walking back to the laundry room, I caught the individual doing this right as he was throwing my wet laundry on the floor. After he left , I opened the dryer he put his clothes in, threw in four reese's peanut butter cups with his clothes, and then cranked it on high and the full time setting.
After that no one's wet clothes were found on the floor in the laundry room the rest of the year.
That's awesome, but I can't help but mourn the loss of four perfectly good Reese's Peanut Butter Cups.
Lived in a bachelor pad that was a fierce party house. Someone kept drinking large portions of my rum, so I started keeping all my booze in my room, and put my name and 'DO NOT DRINK' on a huge cheap plastic bottle of rum mixed with two small bottles of ipecac syrup.
Made it a point to not spend much time around the fridge so the culprit would be exposed. My unemployed roommate started puking several times at each and every party we hosted. Never spoke a word of it to him, just laughed to myself.
Back in high school, there was this f*cker who was a big bully to me. I found out that he has saved enough money to go to Europe for a once in a lifetime event or some sh*t like that.
So the night before he was supposed to leave, I snuck to his house, and placed nails right behind his tires so that as soon as he backed up his car, the nails would go in and puncture his tires. These are hollow nails too, so it was sure to let air out.
He was also the only one at home because his parents had gone on vacation and took car with him so he was left with his car. So you can guess what happens the next morning. I found out he missed his flight because he couldn't get to the airport, also causing damages to his shiny new $4K rims.
On a plane, guy in the seat in front of me--who I'd never met before--starts up a conversation that goes on for too long: at a certain point, he is standing up in the aisle to get closer, continuing to jabber on, while I am trying to be polite and listen.
He continues for so long, that the flight attendant asks him to sit down, since he is blocking her cart and drinks service. He refuses--saying he is talking to his 'friend' (i.e., me). At the end of the flight, he asks the flight attendant for her name, which she provides; he then says he's going to report her to airline management, and file a complaint about her being 'rude' to customers (i.e., him).
Shocked flight attendant asks him if she can get his name, saying it's only fair, because he has hers. She asks him one or two times, then drops it, when the douchebag refuses to identify himself. At the end of the flight, as passengers get off the plane, I fumble with my luggage, so as not to have to follow the douchebag immediately off the plane.
I find the aggrieved flight attendant--and hand her the douchebag's business card, which he had tried to give me earlier. She seemed relieved; I felt profound satisfaction.
I'd been dating a guy for a little over a year when he decided to leave me for this girl he'd been passing off as his 'best friend.' I was always suspicious that there was something between them but they both assured me there wasn't. I was heartbroken. I've never quite been the same after that.
Anyway, I slept his best friend. That was the first time I'd had a smile on my face in months because I knew it would hurt him. And it did. Also the sex was super hot.
For me, the best revenge was on an ex who was emotionally abusive and manipulative. The relationship was filled with gaslighting, verbal abuse, and shame.
The revenge I got was getting out of it, starting my own new life, and falling in love again with someone who knows how to love. Having a great life is the best revenge.
I screen capped photos from my wife's lovers Facebook and sent them to the school administration because he was a teacher and posting pics of your student's test then making fun of them is not cool at all. He was fired at the end of the year.
Edit: After his dismissal, I divorced her and got her removed from her position at a daycare because it was a church based facility with morality clauses for employment. All it took was a conversation with the pastor and she was phased out at the first available opportunity.
Not worst but funniest. I dated a guy for three years and broke up with him when I found out he was married (whole other story). I basically did nothing when I found out other than stop speaking to him.
About 6 months later I was thinking about it and wanted to send a little jab so I logged into the student library site with his student card info and ordered a few dozen books on adultery to be delivered to his office (option available to grad students) which he shares with two colleagues. He's a computer science major so the books were definitely not part of his studies. It was little but gave me a laugh!