If we're going to define sins as "urges that result in actions that result in me doing things that I shouldn't be doing", then it's definitely either sloth or gluttony. Especially if you calculate in opportunity cost; there're people dying agonising deaths every minute, for easily preventable reasons, and I really should be working on helping solve those problems. But, some evenings, I watch TV instead.
If it was my own kid that was about to die due to starvation-induced disease, I'd be working harder to find a solution. The fact that I'm not, just because it's someone else's kid, is a sin. Truly.