I have heard that to "sin" is to sometimes miss the mark, so it doesn't have to have religious meaning, just to show that humans are imperfect and have weaknesses.
Pride, I think. It's a little counter-intuitive, since I don't think that highly of myself I guess, but what other people think, and what I think of myself just matters way too much to me. You could say I avoid things because I'm too proud to possibly look like an idiot or say something stupid. I've just let it control me too many times and instead of swallowing my pride doing something bad to avoid it.
My unmowed lawn and un-unpacked room would also make sloth a close second. ::
I'm probably guilty of all of them but none of them really stand out for me but I suppose pride because I am stubborn at times and I find asking for help difficult. Personally, I always get greed and gluttony confused.