DLC can often feel nickel-and-dimey, where you're paying out real money for cosmetics or other minor additions. For example, Borderlands 2 has dozens of DLC packs that merely unlock extra skins for your in-game characters. On the other hand, DLC can also significantly expand a game's scope, or add a lot more value to the base game. Borderlands 2, again, several additional campaign DLCs that extend the game playtime by hours, with new maps, missions, stories, etc.

I'm curious as to what DLCs out there fit this latter category. Also, for the purposes of the question, assume "DLC" to refer to any official expansion or additional content, even if it's not explicitly referred to as "DLC" by the devs or storefront or whatnot. What are some of the most worthwhile DLCs you've played?