It must be in BioShock. It is ridiculously hilarious - self-ignition for the egg.

In The Witcher 2 Farid and Gaspar's ask you to test a potion for them in Flotsam.
When you import a The Witcher 2 save file in which you drank the potion in The Witcher 3, you can find a letter outside Oxenfurt Academy stating that Geralt may no longer be sterile.
You have to actually import the save file. Simulating a save from TW2 will not make the letter appear.

The line 'It's a material world, and I'm a material girl.' from hookers in Novigrad makes me smile every time I hear it.

My favourite was always a game within a game. I really like Plants VS Zombies inside of World of Warcraft....

MY favorite easter egg is the Cave story reference that I got in the platform game Castle in darkness. You get it at the beginning using the double jump through the clock tower area

The Peacemaker revolver from Battlefield I, the hand tricks are awesome :v

My favorite 'Easter egg' was a cheat code in the Stainless Steel/Mad Doc title "Empire Earth", where to get a mighty pile of free resources you entered in the chat screen, "All your base are belong to us".

I remembered playing "Zero Wing" on the Sega Genesis (the source for the line) in the early 1990s, and that sly reference in "Empire Earth" a decade later was before the meme had really taken off. Good times!

In Castlevania SOTN the game dosen't end at 100%.
If you do some things properly the Dracula's castle turns upside down and you must re-explore it to get to 200%+!

For me, being a programmer, I quite like the esoteric easter eggs that are... not as obvious.

For instance, my very first computer game(s) on PC were Wing Commander and Ultima VI, which I got on CD in '91. And Wing Commander, when you quite, was very polie. "Thank you for playing Wing Commander".

However, what it should have said was, "EMM386 Memory Manager error..."

Ken Demarest posted in the comments of a very old Gamasutra article that this message was something they were fighting right up until the release of the game, and they got around it - by patching the actual error message in the memor manager to say, "Thank you for playing Wing Commander". Every time you quit the game, it crashed on closing - but it didn't matter, because it was very polite about it.

my favorite easter egg, back on the ps2 there was a game called ratchet and clank, and on blackwater falls map, you could do back and side flips, around the announcer and cause ehem something interesting to happen to her.

my favorite easter egg is in hitman 3 where in berlin if you throw eggs at graffiti around the map you get an exit where agent 47 rides a bicycle into the sky referencing E.T. It is my favourite easter egg because 47 is supposed to be a very serious man and i find it goofy/funny

Diablo II – Secret Cow Level! Nothing better.

I'm finishing my playthrough of all the Batman Arkham games right now. There are some great incidental easter eggs and secrets that really fill out the universe. Such as the secret room in the warden's office in the first game. Scarecrow's secret research room in the second.

But if I had to choose, I think the calendar man rhymes in Arkham Ciry are the most impressive amount of secret dialogue that you can find. With a different speech for every holiday and a special one plotting Bruce Wayne's downfall on the anniversary of Rocksteady's founding.

Great character moment and incidental storytelling that is missed by 99% of players.

All time fav secret
In Ultima IV spent all day in the final Abyss dungeon - In the end you are asked several riddles. When the final riddle was asked my friend and I had no idea what the answer was. We called up a friend who thought he knew so we entered an answer. Then bam kicked out of the dungeon. When Ultima V came out you meet a horse name Smith who ends up telling you he forgot to tell you something important and would be the answer to the final riddle in Ultima IV


Additional favorite secret would be - Super Mario Brothers negative world -1 (minus worlds)
Favorite Easter Egg - Cadbury Easter Cream Egg

I'm a simple man, any easter eggs related to Dark Souls, like in Fallout 4, Witcher 3. etc you find a bone fire or something related with Solaire. Not only because I'm a huge fan of From Software but also because I do believe that Hidetaka Miyazaki is the Dali of this Era in the video game industry and I'm pretty sure that the most devs have a huge respect for his work.

My favorite easter egg was probably from Battlefield 4's phantom program. It was super involved to do all the steps but it was fun looking for all the clues and using real world information to figure out. I would watch streams of people looking for any info and suggesting ideas and once the steps were solved you had to do missions in the game. Usually an easter egg is a one and done thing but this lasted the whole life of the game with the dlc's Dice put out, and the missions actually forced me to be better at the game since each mission forced you to do something like get a lot of kills with the tank or jet which I was not very good at to start but I became a master at lobbing tank shells across the map which was soooo satisfying. It was just a lot of fun and I got to have some exclusive items for a time.