I love them all, but will have to vote So Jealous since that was my introduction to them - well technically a HS friend of mine was really into If It Was You, but I was still in my ska phase then. Checked out So Jealous after they covered Dancing in the Dark for the Bruce Springsteen “Hanging on E Street” series and I think Brian Fallon shouted out So Jealous in an interview as an album he was really into at the time. The Con would probably a close second (and on many days could also be my favorite, so hard to pick)