Hardest Achievement/Trophy you have ever earned.

[Misha]

Nothing to shake a stick at but I would have to say any of the easter eggs I completed on black ops 2 zombies. I'm still working on doing the Origins one on solo

[Mardan]

Any of the neopets game avatars lmao
If we're only talking about console games though, probably the max killstreak one in Halo 3.
Getting it legit felt amazing.