The best visual gag in any piece of media was the giant vault door concealing a normal-sized door in Portal 2
Actually the best joke of all time period was also in Portal 2 and it’s when Glados says “Well, this is the part where he kills us” and Wheatley follows up “Hello, this is the part where I kill you!” and the chapter card pops up reading “Chapter 9: The Part Where He Kills You” and then you unlock the achievement “The part where he kills you” and the achievement description reads “This is that part.”
Portal 2 is a fucking masterpiece for hundreds of reasons