I Would Have Been Your Daddy Skull
The I Would Have Been Your Daddy Skull is one of the most annoying skulls to get in the entirety of Halo 3. It involves a part on the level The Covenant which comes in at the very end. The end of the level consists of a long bridge with seven light rings above little platforms you can jump on. To receive this skull, you have to jump in the light rings in a consecutive order. The order is 4 6 5 4 5 3 4 and once jumped through the rings will begin to flash down the row from 1 - 7.
When the game first came out, Bungie taunted the people that were looking for this skull. It was eventually found through a lot of hard work, even to the point of looking at the coding in the game itself. It was a challenge to the community to finally find the skull, which they did. Originally, the rings that had to jumped through were double that, but finally it was found that it needed to only be the last seven notes that mattered at all.
Regardless of all the back-story, there has been a line relating to I Would Have Been Your Daddy in all three of the games. It serves the same purpose as a skull as it did in Halo 2. It unlocks harsher dialogue from enemies as well as some odd dialogue from your UNSC back up. I believe a marine once shouted "This is SPARTA!" as he chucked a grenade at an enemy.