For me, playing Metal Gear Solid on the PSX1 and spending a good hour or so running around and searching for the CD case to contact Meryl. When I was about to finally give up and read through the manual for the 4th time, in a desperate bid to find an answer, did I finally spot on the actual case of the game the code i had been looking for.

There is a female assassin in "Hitman: Blood Money"'s mission "You better watch out..." which directly recognizes agent 47 and tries to lure him to kill him.

The many items in Oblivion that are all references to my favorite franchise the lord of the rings

As a few others have already posted, I think the best easter egg is the first: Adventure on the Atari 2600. Others have not yet pointed out, though, that It was groundbreaking in being the first easter egg, but also depressingly prescient (depresscient, if you will) in its motivation. Long story short, the head of Warner/Atari at the time was your classic business school MBA piece of crap who, like most executives in Hollywood, actively resented those with actual talent. As Atari developer Larry Kaplan recalled way back in 1983 (www.gameinformer.com/b/features/archive/2013/02/26/activisionaries-how-four-programmers-changed-the-game-indu stry-forever.aspx):

“Kassar called us towel designers,” Kaplan told InfoWorld magazine “He said, ‘I’ve dealt with your kind before. You’re a dime a dozen. You’re not unique. Anybody can do ­a ­cartridge.’”

My god. The unmitigated disrespect, contempt, and stupidity. We are still dealing with that today. The video game development world is much larger but still rife with disrespect and scum-of-the-Earth business types like Bobby Kotick and John Riccitiello. This easter egg didn't just signify the start of modern games, it signaled the start of the game industry itself, in all its manifold forms.

finding something of "value" in the game {like "GUNS in gta} --- {toughness ; if some one 'shoot/attach' you...you WONT DIE as quick}

You should also try it on GOG's mainpage. ;)

Hero's Quest 2 Trial by Fire. When you learn you can continually sell the Dervishes beard to the gnome at the magic shop. Also when picking your nose (lockpicks) you can unlock all of your potential (death)!

In the Secret Societies games mode for Civilization VI, one of the Relics that can be generated by Cultists is a Strategy guide for Civilization II

the red stapler and TPS reports from Office space on a random desk in the office building in F.E.A.R. Made me laugh out loud during a really tense section of the game.

Interesting, I adore Fallout New Vegas, I cannot get enough of it. Playing with the Wild Wasteland trait brings a lot of great easter eggs and references to pop culture, almost too many to list.

At the end of the game whilst at the battle for Hoover Dam, changing to the NCR emergency radio, there is occasioinal chatter including the lines "they're coming outta the walls, man! They're coming outta the damn walls!" and "game over man, game over!".

This being a reference to Aliens, one of the best films ever made. Bringing two of my favourite series together like this never fails to make me smile when I hear it.

Luke Skywalker in Duke Nukem 3D (A Force to be reckoned with) Secret/Easter Egg Episode 2 Level 8 (Lunar Reactor)

Quake was the best for me

My favorite cheat of all time is probably in Warcraft II when entering the "cheat" code: disco...
When I found out that it was possible to play this track on a CD player my 12 year old self was so happy :)

Close second is Radio free Zerg from Starcraft.

"THAT'S ONE DOOMED SPACE MARINE"

To me it's the fart sound in Full Throttle.

If you type in a certain combination into the safe, you hear a fart noise. I love fart jokes. Who doesn't?