I quit playing seriously a while ago. Now my friends and I just troll the other team with spy or engi spam.
Best was when an IRL friend, KKR, and I all built our buildings in the other teams intel room on 2fort. One of us grabbed the intel, but then just stayed with our engi nest, so our team couldn't capture it. We had teleporters to and from or respawn area, we dispenser and sentries guarding every entrance. It was set up in a way that even an ubered player couldn't get in, because of all the knock back from the sentries, so they could only ever see and destroy one sentry. Which we would quickly rebuild. We're all good engies, so the few spies that did get in got killed quickly without doing any real damage.
Even though the enemy grabbed out intel quite a few times, they couldn't get it back to their own room, and because one of us was always carrying the intel while maintaining the nest, the game went into overtime. At this point, everybody left the server except us 3 engies, and 4 players on the other team. They need got through, and eventually the game went into Sudden Death. 3 engies is a shitty Sudden Death team, so we lost.
Still, best game ever.
Another time, the same friend and I went spy, equipped the Cloak and Dagger and the dapper that reverses the building process, and trolled the other teams engies the entire game. Don't remember what the map was called, but there's a big open space around the intel, so there was plenty of places to hide. One of use would sap something while they weren't looking, so the engies would rush over there to knock it off and try to find the spy. Meanwhile, the other would run to the other side of the nest and sap a few things. It'd just go back and forth. Eventually, they got 2 pyros to camp the place, but we were good enough to get caught more than once or twice. At one point, a Heavy joined, so we had effectively taken half their team out of the game, looking for two spies