What was the most memorable prank pulled on Friends?

In the popular TV series Friends, one of the most memorable and iconic pranks was when Ross Geller played a joke on his friend Chandler Bing. In the episode titled 'The One with the Embryos' from season 4, Ross and Rachel bet Monica and Chandler that they know more about their friends. They proceed to ask a series of questions, with the winning team getting to keep the apartment.

One of the questions Ross asks is what Chandler's job is, and when Monica and Rachel don't know the answer, he declares himself the winner. Chandler, who is visibly upset, tells Ross that his job is a 'transponster' which Ross accepts as the correct answer. Later on, Chandler finds out that there is no such thing as a transponster, and Ross reveals that it was just a made-up job to win the game.

The prank has since become a fan-favorite, with many fans of the show still talking about it years after it aired. It has also been referenced in various other TV shows and movies.