Not really an Easter egg, but I remember in Ratchet and Clank: Going Commando, you got news the villain was on another planet. You arrive at said planet just as the villain is leaving. Clank then says “Why is it, that we are always too late?” then they both look at the player. Best scene in a game if you ask me.
I remember in Cod:ghosts extinction, if you shot the hotel sign with a sniper everyone got a rainbow text saying LOL in the middle of the screen and then aliens explode into little plush aliens. People would always think you were a hacker lol.