my easter egg is the scarab gun 
probably because i barely got it last year after 6 years of attempting to do it!
Scarab gun was cool, also like the caveman family just because it was so random.
mine was the grunt at the end of CE hiding during the escape.
sword rex in halo 2
The Red Vs. Blue thing in Halo 3 was great and all, but nothing can beat the scarab gun. That gun was just pure awesomeness.
Ape figure in ODST
Rex sword back in H2
The easter eggs with the Red vs Blue guys aka Rooster Teeth, as the marines on Halo 3 with their Password conversations
Brute DJ at Club Errera in Halo Reach
Tribute Room in Halo Reach
The lonely grunt at the end of the game in Halo 3 just before you jump into the Forward Unto Dawn
Red vs Blue Easter Eggs in Halo 4