I saw one of these for FFXIII and thought a general one would be good.

The one that was hardest for me was the 5 kills, 5 weapons in Modern Warfare 2. That one took me absolutally ages. Stupid thing is that in teh end I got it entirely by accident. Go figure.

Another one that took me a long time was the all endagames achievement in Dragon Age: Origins. I learned in the end that there was a glitch that needed to be sorted via a patch. That's hours and hours of my life I'll never get back.

So what achievements did everyone find hard? If you don't go for achievements that much then which, that you've seen, sounds the most difficult.