I always found it great to see Sam and Max in Lucasarts games released before their own game.

Pure awesomness when seen for the first time - especially close to the date when it was implemented into game itself - Horde Elite Chieftain via World of Warcraft! Named after a real-life group made of Blizz crewmates, it's a hidden gem of amazing rock'n'roll inside Azeroth. There were two places where you could hear them and see them playing in-game. Trust me - without knowledge, it could blow Your mind for some time! If You dont know this gag - go and find it @ Youtube. For the Horde lads!

i always belived in the doom game 1995 that it will be a legendary series and it have thats a win for all payers!

Trackmania, a Royal map with farming theme... Instead of completing the track, you may drive your car at full speed precisely against a solid harvester (which is seems to be only a part of decoration), fall under track level, get a jet boost and fly through a secondary finish gate...

Soo many good ones have already been mentioned!

I'm going to share a weird Easter Egg I stumbled upon in walking simulator Gone Home a while ago:

There's a little basketball hoop hanging on the door in Sam's bedroom. So, when I spotted a tiny little basketball stuck in the ceiling of the garage, I decided to try and get it so I could play some mini basketball.
It was a bit of a struggle to get it loose, but I did it and returned to the bedroom and after a few tries, actually managed to get the ball through the loop :D

I did NOT expect to get any reaction from the game, but I did in the form of a truly bizarre cat video, of all things.
It somehow felt both random and very fitting at the same time. Mind you, I was playing late at night and was quite tired at this point, so that might have helped me get over the immediate what-the-heck-ness of the moment.

You can watch the whole thing for yourself here, if you'd like (not my video): https://youtu.be/GwEGPdIsHnY?t=206

It was an enjoyable 'easter egg' for me when the lovely Triss showed up at Corvo Bianco in Toussaint after I finished the 'Blood & Wine' expansion pack. What a pleasant way to end an adventure. Kudos to Project Red!

For me it will always be the minecraft easteregg in Borderlands 2
Not are both of them my most played games you even get a nice Weapon and Skins :D

While there are many that I enjoyed I think my favorite easter egg is the Dog Ending from Silent Hill 2 not only because of it being wacky but also because of a real life story.
Back in high school two of my friends were in a heated argument about how many endings there were, both having finished the game twice already and were on their third play through but being curious they went to the internet, which back them was not as readily available as it is nowadays, and one of them read there were 4 while the other that there were 5.
After listening to them for quite a while I said, with as serious a face as I could since I had no idea, you're both wrong there are 6 endings, they both stopped and started asking me how could I possibly know since I had just started playing the game, to which I replied simply with a still serious face, you'll see.
Later that week they found out that there were 6 endings and were freaking out about how I knew of the 6'th secret ending, the Dog Ending, even though I had no internet and had not visited an internet cafe, while all the while I had a huge grin. :D

From the game Fallout New Vegas, there are a series of Easter Eggs that are part of hte Wild Wasteland trait. I find all of these easter eggs to be enjoyable, but out of all of them, my favorite one is the "Indiana Jones" easter egg where you find a skeleton and a hat inside an open refrigerator. This is making fun of and paying homage to the scene in the movie where Indian Jones rides out the nuke in a lead lined refrigerator and survives, which is actually rather ridiculous. Some people think of it as a 'dark' easter egg but I just think of it as poking fun of a movie scene which coined the phrase "nuking the fridge" in response to its ridiculousness.

My favorite is still John Romero's severed head being the true, secret final boss in Doom II. Silly? Yes, of course it's silly, it's Doom. But it's also just fun, backwards voice and all. And it allowed for some freaky-cool challenge runs that were much harder than the intended way to play (even if they were technically cheating).

All of the minecraft stuff in borderlands. Was cool looking gear and weapons and easter eggs

My favorite easter egg is kind of cliche because it's often credited as being one of the first and is also very well known in pop culture. And for good reason.

The easter egg is William Robinett's signature in a secret room in the Atari game Adventure.

It is my favorite because of WHY he put it in the game. As an uncredited developer under the thumb of Big Corporate he coded what is basically a middle finger to The Powers That Be. It is radical. Subversive. Most easter eggs are plain fun or amazingly clever... but when you see that MADE BY WILLIAM ROBINETT flashing on your T.V. you are part of something big. Meaningful. Proof that the little guy can always get one over on The Man.

If you manage to glitch out the game in Baldur's Gate so that an actor scripted to say a line isn't present, Biff the Understudy appears to take their place.

The easter egg gets kind of meta in Baldur's Gate II, since he appears during a quest to, you guessed it, fill in some lines for a missing actor in reference to the easter egg.

YEah that Duke nukem 3D "You are not supposed to be here" in a cave that you can only reach with invincibility cheat PLUS unlimited jetpack cheat (DNCORNHOLIO and DNNUJ)

Oh the memories, playing in my full-bastardly-tuned epic 486 dx2 witha full blown 2 mb of ram amd SUPERVGA monitor, and a super duper soundblaster pro.

creadits to Zaknafein on Steam forums, who posted the capture.
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Trials Evolution and the secret that is revealed in the year 2113 under the Eiffel Tower