What's your favourite video game Easter egg?

With Easter coming up, and Discount Easter egg Day soon after, I'm curious: what's your favourite Easter egg in video games? I suppose this world must house some edutainment game which would let you answer this literally, but I'm more interested in those hidden jokes, references, and oddities which surprise and delight. Tell me your favourite and who knows, maybe I'll eat a Cadbury Buttons egg in your honour. Next Tuesday, once they're cheap.

Myself, ah, I don't know about a single favourite but I do very much enjoy the Easter eggs in Battlefield games - even if I don't much like the series myself. They've snuck in surprises like gigantic sharks and finger guns. Always enjoy seeing them.

As a certified old person, I do also have a soft spot for Doom II's final boss, the Icon of Sin. Turn on noclip mode and walk into this giant demonic head, and you'll find yourself inside with the severed head of former Id Software level designer John Romero (who wasn't actually the mapper behind this level). And the demon's garbled greeting is a distorted backwards recording of him saying "To win the game you must kill me, John Romero". Shoot his head, and you will. An oldie but a goodie.

All sorts of other strange and wonderful things have been hidden in all sorts of games, secret levels and jokes and borrowed characters and behind-the-scenes peeks and goodness knows what else. What's your favourite, reader dear? Always happy to hear about neat things I might have missed.