Definitely Headhunter. Getting a headshot on that assassin that constantly walks up the stairs seemed very difficult to me. Even when I adjusted for gravity drop, it would always kill the guy and not award me with a headshot.
Daredevil would be the second hardest because it is extremely dependent on where the checkpoints spawn at. Speaking of the Bonfires challenge, getting three stars on the expert version of it was also frustratingly hard. Nothing more fun than getting sucked in by their magic when you're close to achieving three stars.
163/164 games completed (including DLC)
Completion percentage: 100%
Latest retail completion: Dragon Age II
Latest arcade completion: Dust: An Elysian Tail
Currently Ranked #1 on Achievement Completion Percentage Leaderboard on TA