I would say pride is the worst from how proud religious people are about their beliefs. "All is vanity!"
Pride (vanity) — A desire to be important or attractive to others or excessive love of self (holding self out of proper position toward God or fellows; Dante's definition was "love of self perverted to hatred and contempt for one's neighbor").
Pride is one of the worst sins on this list because it taints every virtue, and makes salvation much harder to obtain. Although the seven virtues counter each of the deadly sins, pride is the sin which is able to affect them all in a way that the others cannot. Go through the list of virtues, and realize how horrid it is to mix in the sin of pride.
I think love should be the worst sin for all the pain it causes.
I still have a broken heart after twenty years.