20 Percent Cooler is probably among the longest lasting memes, (if not, THE longest lasting meme) in the entire brony fandom.
Where did it Come From?
During episode fourteen, "Suited for Success", Rarity offers to make each of her friends a fancy dress for the Grand Galloping Gala. After her first attempt, her friends reluctantly express their dislike with the designs of the dresses, and Rarity then remakes each one of them in exact accordiance with each of her friend's wishes. During the song, "Art of the Dress," in which Rarity is making the dresses, she turns to Rainbow Dash, asking what changes should be made, to which Rainbow Dash replies, "It needs to be about 20% cooler."
Where is it now?
One of the most common memes on Ponychan, 20% cooler is used as an approval gesture in threads. It can most often be seen posted after a picture of Rainbow Dash is added to thread, because posting Rainbow Dash in a thread makes the thread "20% cooler." It's lost most of it's humor and has become a standard phrase for the most part, though it's become somewhat less frequent over time.