This is a trick question. I'm the only person powerful enough to keep me from achieving any goal I set for myself. Since you've said you can't pick yourself, there's no one left to list. Any person I tried to blame would just be an excuse to not blame the real obstacle, me.
Goal - complete a software requirements specification for a database application.
Idiot - the client, who won't answer emails or show up for review sessions. Later he'll complain to his cronies about how the IT department screwed it up again, and gave him a system that he can't use.