There is a big shark in Battlefield 3. And Battlefield 4 too ! JAWS COMING !

In Mafia, you can get red Bolt-Thrower from the end of the game. You have to shoot into windows in different city places for this. I did it so often that memorized this sequence after all))))

In Curse of Monkey Island at one point you can poke your head through a hole and end up in Monkey Island 2 - LeChuck's Revenge.

Have you heard about glitches? It can happen in every game.
Information is based on electrons, and these are under quantum and not classic physics laws.
That means that there is a moment/possibility/statistical assurance that some of the information will be wildly transformed in a different place allowing for teleportation of the main character or any movable object in a game.
That means that now and then, no matter how programmist will try, you will go through walls, floors, and ceilings, dematerialise, jump further, or time freeze while you trying or not intend to do so.
There is more, occasionally you might experience glitches in real life, this is where time slows down, or juggle many things, or near missing something, your attention is quicker than laws of "classic physics" allowed.
Going quantum is my favourite secret.

Doom II, secret level two, MAP32 "Grosse" now renamed to "Keen" in which Commander Keen, or rather multiple Keen's appears as a cameo which interlinks or connects the universes, though it is connected to the *pre-reboot and Doom Marine being a likely Keen descendant. This to me is the greatest Easter Egg in gaming and wish to see it explored in the Slayer universe, and was appreciated to experience id bringing this about. It may also give some background to Keen which is up to the player to digest.

*The Keen skull with the helmet and Keen’s helmet is a reoccurring theme in Doom 2016/VR and Eternal. In addition, Keen sweets appeared in Doom Eternal.

All which emphasises the idea that the universes are shared, so without a doubt MAP32 is the best Easter egg. Just realise it already id!

My favorite is probably one of the first ones I ever found. This sign in GTA3 always gave me a chuckle.

Thief 2, Mission 3 "Framed". If you toss a Scouting Orb over the left wall at the mission start, you'll see a pair of dancing zombies.

Dunwich building in Fallout 3 really cool with actual worship site and really cool audio logs, weird scenes etc.

The hanging Keens in Doom 2.

It was the first time I ever really encountered that kind of intra-company "cameo", and I remember having so many questions: why would Hell taunt Doomguy with visions of Commander Keen? Those bodies had to be fake, right? Who put them there? And why in a map based on Wolfenstein? Many years later I found out it was just Adrian Carmack being petty, but still, that's the first easter egg that got me hooked on the idea of shared universes, crossovers and connected stories in games.

It's easy to take it for granted these days, the transmedia age where River City Girls and Double Dragon are in the same world, 2064: ROM and VA-11 HALL-A, Unfinished Swan and What Remains of Edith Finch... but here's where it really started!

Curse of Monkey Island where animated Guybrush meets oldschool Guybrush from Secret of Monkey Island

So, interesting Secret: World of Xeen has an emergency back door.

If you ever loose the Sword of Xeen a weapon REQUIRED to win the game, just go up to any magic mirror in game, and for your destination, tell it "I lost it". The mirror will hand over a new Sword of Xeen

For extra fun, the game does not check if you actually lost the sword or even found it to begin with. allowing you to have multiple Swords of Xeen as soon as you find a magic mirror.

In Star Wars Dark Forces II: Jedi Knight you can encounter Max (from Sam & Max) in one of the houses of Barons Hed level. The house is locked from the inside and you only have a chance entering it when one of its inhabitants opens the door and leaves the house. Max is well armed like a psycho he is, and it is the only time you could see him in full 3D during the 90s, was really amusing for me as a kid to discover him.

Nothing quite comes close to the elaborate set of secrets across multiple games that made up the "Eye Sigil ARG". The Frog Fractions 2 devs got a bunch of other game devs to hide a sigil and a piece of a map to their games, with huge community effort to decipher the whole thing.
It didn't even seem to start out as a Frog Fractions 2 thing, it just seemed like a bunch of entirely unrelated games had all added this sigil with no further context, until people started also finding the map pieces.

mine has to be the secret world codes in terraria. my favorite of them has to be for the worthy the ramp in difficulty is really hard but o so rewarding.

the one i found when i was young on Broken Sword 2. in the metro, you found Robert from Beneath a Steel sky and is very weird i love it