Mile High Club is one of my proudest achievements. I was so happy when I finally got that.
Beating Mass Effect 2 on Insanity also ranks up there. Fuck those Scions. Fuck them hard.
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Also, beating COD: WAW on Veteran ranks up there too. Not because it was particularly hard, but just because of how ridiculously cheap it was. 7 grenades in your lap from 250 yards away, enemies that would rather shoot you at 100 yards than your teammates at ten feet, infinite enemy closets, etc.
I don't think I've raged more during a game than during World at War on Veteran.