What is the hardest achievement in Halo 3

The hardest is probably the recon armor and this is because you have to be or know people in bungie. The other way is for something weird to happen such as the traffic cone kill.(look it up on google or youtube)The final way would be to make lots of videos.

Overkill and Famine achievements are the hardest I think... that and completing the campaign on Legendary difficulty.