Gotta be the Easter Egg from Pathways into Darkness. On the level that you finally find Muller, there's a secret wall. Follow the hallway behind the secret wall, and you'll encounter an object called "Easter Egg".

If you eat the egg, you're instantly teleported to the last level of the game.

How many other games make their easter eggs ACTUAL easter eggs?

While there's hundreds of easter eggs in the Witcher series, the one I remember the most was the DRM "Defensive Regulatory Magicon," and of course the solution is "Gottfried's Omni-opening Grimoire (GOG)." It was so clever and unexpected and hilarious.

Max Payne 1, walking out onto a ledge through a broken window. I found myself in an abandoned hotel room with a dead mafioso. He had been staked through the heart, the person responsible left their name written in blood "Buffy", I can only assume they must be some kind of vampire slayer.

Jill’s Sandwiches ​from Dead Rising
In Capcom’s survival horror Resident,lead character Jill Valentine is nearly squashed when she’s trapped in a room with an ominously lowering ceiling. Barry Burton rescues her just in time, though, exclaiming: “You were almost a Jill sandwich!” The developer chose to celebrate its mastery of dialogue with this hilariously named burger joint.

its really hard to say what my favoret easter egg is.
i never really put thoght into it.
the black isle/ obsidian fallout games are full of easter eggs from pop culture
fallout 2 litteraly had a easter egg
so that that easter egg wins because it a litteral easter egg

The best one for me was Duke Nukem 3D's Doom the Space Marine corpse. Every action game at that time was trying to be the doom killer but none of them took it that literal. Also in a deeper meaning, Duke destroyed all the cliches (serious honorable hero, saving the world, constantly in indoors in space for some reason etc.) that Doom set for that genre. So Duke becoming the doom killer was natural. This easter was the perfect symbolism.

I'm still waiting to find an Easter Egg that has not be discovered before!

Once upon a time, I decided to buy as a birthday present for my kid, aged 10+ at the time, a video game. It so happened that I chose Dungeon Keeper. Now my son was born on November 22, but he only got around to playing the game on December 1st. (I, on the other hand, was already exploring it- adult privileges :P - and gradually becoming quite obssessed with what you could do in the game.)
Anyway, to make a long story short, my son started playing in earnest on December 1st and' lo and behold, the very first intro screen displayed that day the message "Happy Birthday to you". None of us knew it at the time, but we had stumbled across our very first easter egg, in this case a hidden birthday message for some of the devs of Dungeon Keeper. I will never forget the look of wonder on my kid's face, while my own jaw was hitting the floor. It was, of course, a coincidence, but truly one of epic proportions and cosmic synchronicity.
Later we came upon the second wonder: on full moons, a secret level would appear on the world map. Given that we usually played late at night, one can imagine the feeling ;) . And it was oh so subtle: the level's flag didn't simply came into existence on the night of the full moon, it gradually gained consistence over several nights.
So, for me, the secrets and easter eggs of Dungeon Keeper, one of the best games ever, are memories that we will never forget and always cherish.

The add texture here easter egg in the hitman games

SimCity 2000

If you were feeling particularly mean-spirited you could shoot down the helicopter by repeatedly clicking on it. His commentary as he crashes is hilarious.

In Fallout 4, after meeting the Brotherhood, you're tasked to retrieve some junk items.

One of those, called the "flux sensor", has the serial number CM-88B 180924609 when inspected in the inventory and is a reference to the first Alien movie. The Nostromo ship in the movie is a CM 88B model with registration number 180924609.

The best Easter egg for me was in Deus Ex 2 Invisible War. At the end of that game you had the option of choosing 4 different sides each with their own ending but it tasks you with killing some characters. But there was a secret 5th ending where you don't have to kill anybody. If you carried a Unatco flag into the restroom and flushed the toilet then you would be transported to a nightclub where all the game's characters would be partying. And if you played a non-lethal playthrough then every character that didn't die would show up to party. It was the absolute pacifist ending.

I'm personally a big fan of Fatum Betula's "PNG of a Baby being surrounded by a biblically accurate angel inside of a sunken church" easter egg. It just kinda hangs out every so often, and really fits with the strange tone of the game. I first saw it when i filled the ocean with wine, so everything was red. Tricked me into thinking i did something haha

Playing the duck in Duck Hunt (NES).

I think my favorite easter egg is the giant teddy bear in Fallout 3. Every playthrough I would go to "SatCom Array NN-03d" (yes, I had to look up that exact name) and throw grenades at a door to get the big bear to come out. Although now that I play on pc, I can just use console commands to get through the inaccessible door because relying on the physics and the grenades was tedious.

But in the end I take the bear and put it on my bed in Megaton. Its lovely trophy decoration for any completionist's home.