1. Gluttony
Crowley finds Gluttony the easiest sin to Tempt humans into committing. It's mostly his fault stuff tastes good. All that "synthetic" flavouring that he gave mankind the idea of. It's delicious.
Crowley's favourite part, though, isn't that he invented synthetic flavourings, or that he's the reason so many humans are Tempted into committing Gluttony. No, Crowley's favourite part is that Aziraphale loves candy bars.
Crowley buys him all sorts, "just to be nice." He knows Aziraphale falls for it. Aziraphale always does. And he eats all of them. And he eats the chocolate cake Crowley brings, and the hot chocolate with extra marshmallows. And he eats the crumpets, and once Aziraphale even let Crowley cook him breakfast. Crowley made pancakes and French toast and eggs and bacon and oat meal and cereal, and Aziraphale ate it all.
Sometimes, Crowley very much loves his job.