My biggest achievement in a video game is with the game Sea Of Thieves, the achievement is called "Legend of Glitterbeard" you need 8 players to tap on the tree at sunset precisely, the complicated thing is that the maximum number of allies on a ship is 4 people and you would need 4 more to participate and since it is a game that is about attacking everything that moves it was almost impossible to achieve, until that day there was a community event and when we finished it there were too many ships together (more than 12 people) and a crew told us to follow him and we all went to a tree... I understood what they wanted to do and I gladly joined the achievement, the sunset was so wonderful, the tree lighting up, the song in the background and above all that server full of pirates joining for the first time for that great achievement! For me, it's one of the best achievements I've ever had, and then there's finishing Dark Souls, haha.
(I'll share a bit of the video; I couldn't upload the whole thing because it's over 3 minutes long.)