Well, my favorite easter egg was revisited today in the morning, and is the vic viper orbital frame drived by Leo in Zone of The Enders The 2ND Runner. Hideo Kojima's underdog underrated game.

That's because the orbital frame he drives is basically ripped from Gradius a konami shmup from the 80's but with a twist. Now it can transform into a robot, and is a boss fight you have to defeat in the middle of the game progress.

The first time i saw that comming i couldn't believe my eyes! because it was not just a simple easter egg you have to find in an obscure way, it was a freaking Easter Egg Boss Fight!

Mine's very NSFW, so I hope that's okay, but one of the first easter eggs I ever experienced was in SiN. There was a secret room you had to noclip into that had a camera feed of the main antagonist... uh... getting hands on with herself in a hot tub. The video feed only showed the back of her head and played audio, but if you noclipped a second time, you could actually enter the room she was in and see the animation from any perspective. It was great! She was in a bikini, by the way, so no nudity, but very suggestive animations and audio.

Look, I was a kid at the time, okay, and a nerd who was never very lucky in love, so experiencing some polygonal titillation was a big deal, especially back in those days when sexuality wasn't really a thing that games explored. It blew my mind in a way that kids these days and in the future won't be able to experience thanks to how easily that kind of content can be accessed now. So that makes my experience kind of special, I guess.

Oh there are so many... But probably the most rewarding and pleasing one (although not at all hard to get) was meating Blind Guardian and solving a quest for them in Sacred 2. And after finishing the quest you get the best reward ever given in any RPG - a visit to an in-game Blind Guardian's concert. Nothing beats that... Not even Dandelion's hillarious performances in the Witcher series.

Another favorite of mine is in StarCraft where even the cheatcodes were Easter eggs like "theresnocowlevel" for instant victory. But this is not the easter egg I mean. The one I am talking about is the pilot of the shuttles of the Terran race: their icon looks very much like the pilot of the landing vehicle in the movie Aliens and their lines that they repeat whenever the player commands them in StarCraft are actually the few words said by the pilot in the same movie before she was killed by the shuttle crush caused by xenomorphs.
And to finish, this is not the first time Blizzard did this, because my third favorite Easter egg is again related to sound of units but this time in Warcraft : When your Gyrocopter is shot down, it's last words are "They came from.. b..ehiiind...", which is a reference to the battle for the Death Star in Star Wars IV - A New Hope as those are the last words of Gold Five when he was shot down.

The weeping angels in Witcher 3 is an excellent Easter egg for Halloween. There are so many that I have lost count since I have been playing games since the days of Infocom.

Struggling to think of any major easter eggs, but I did enjoy spotting Gavin Thorpe as an Empire hero in Total War Warhammer 3 (he was the head designer on multiple editions of the Warhammer tabletop game, as well as writing a lot for the Black Library)

David Doak from Rare / Free Radical being used for a number of the in game models in Goldeneye 007 always made me smile too.

Mine would be from the OG Adventure from the atari 2600

Just because you have to go through a convoluted mess of a process and the reward!

One of the first credit screens in history!

Konami code!! Do I really have to explain?

My favorite Easter egg was finding a teddy bear sitting on a tree stump or on the ground in resistance fall of man. You can only find this in MP by glitching off the map with friends. This was a confusing and interesting thing to see. haha

El mío es cuando estuve jugandoThe Witcher 2: Assassins of Kings, cuando fui por un lado de la torre y encontre a Ezio Auditore muerte a lado de un montón de paja.

My favorite easter egg by far in any "video game" (using a very wide meaning of the term, being a game played with the assistance of a computer - in this case a text adventure akin to venerable titles like zork, the hitchhiker's guide, and ancient MUDs), was encountering Morte in the epic "Enter my dungeon" gog community rpg/giveaway run by Doc0075 in late 2019. My character, the Dwarven brigand Og'rialt, in the process of fleeing along one of the two southern tunnels while fighting various monsters with a band of adventurers, foolhardedly entered a swirling magic vortex, only to discover that it was a portal. On the other side of the portal, he encountered a flying skull, who called him "Chief" and asked if he'd gotten shorter since they last met.

That first encounter happened via private chat, so sadly I can't link there, but as a huge Placescape Torment fan, you can imagine my absolute delight in being the sole lucky player of that game to randomly encounter Morte and gain various boons from him. The skull gave Og'rialt a special book and mirror, which ((many) weeks later) turned out to become small portals as well, allowing Morte to fly out of them in order to take down a Hag and a Beholder, two amazingly powerful monsters, which the large band of adventurers fighting through the southern tunnels had otherwise little hope to overcome.

A year later, in a not entirely unrelated giveaway also run by Doc0075, I reprised my role of Og'rialt, and Morte once again showed up to save the day.

I still start smiling just by thinking about how much fun that all was. :)

There's a sepcial encounter in Fallout 2 that lets you go back in time and break the Vault 13's water chip, which sets Fallout 1 into motion.

Mass Effect 2's Arrival DLC.

There's a countdown of 2 days before the Reapers reach the Milky Way and you have to destroy their means of fast travel between the solar systems before they arrive. If you wait 2 IRL days for the countdown to end, you will reach a very creepy game over screen where the Reapers harvest all intelligent life in the Milky Way.

Ah a tough one. Grand Theft Auto San Andreas i used cheats for moon gravity and flying cars to fly up infinitly, you had to bump a vehichle and then enter it in the game to fly up to a small easter egg city of gta 3 which is a little above the sky limit. Some areas are walkable there but some you fall through.
There i don't know anymore if cars driving above in the small portion of gta 3 easter egg city.
Can't forget this detail since i played the game long ago on my pc.
Edit:
I could not confirm the cars being in the sky city so i might me misremembering that one

In CONTROL- toward the end of the campaign, the main character Jesse reacts to a song by 'The Old Gods Of Asgard', and says something like "oh, i love this band"

this ties CONTROL into the ALAN WAKE universe- a game with a heaping colleciton of its own Easter Eggs- all of the references to Twin Peaks, the Twilight Zone, Stephen King novels, and many more. In ALAN WAKE, you meet the retired musical duo 'Old Gods Of Asgard' several times, and they help you in some interesting and humorous ways. Their whole situarion is a deep iceberg- the band members names are Tor (Thor) and Odin, and lightning does seem to accompany them, wherever they are- suggesting that they are in fact the actual mythical figures of their namesakes.

both CONTROL and ALAN WAKE are full of up-front references, containing deeper suggestions of what the stories universe may contain. i find it very satisfying how CONTROL doesnt make you struggle or wait around to see the interesting and ethereal, but hands it right to you, and says "take a closer look".

"What is your favorite video game secret/easter egg?"

The answer that comes to my mind is Heretic's cheat code / easter egg "iddqd". I remember using it for the first time back in the day, desperately in need of "god mode", just to immediately start laughing out loud after being instantly killed because I realized the developers were secretly making a reference to Doom, a game that I play to this day. Do I need to tell my reaction when later on I tried to use "idkfa" after loading my last save game? Such wicked and memorable punishments for cheaters!