Similar to Best Friends Forever (BFF), Best Enemies Forever (BEF) means two people who love to hate each other and rip on each other every chance they get. They are sworn enemies who form a bond of hatred so strong that it's more like a perverse kind of friendship.
Bill: "Goddammit, I hate that asshole Craig! Who does he think he's trying to impress??"
Dan: "Dude, will you stop talking about Craig? When did you two become BEF?"