I have to go old school.
As much as i like Vanilla StarCraft, I'd have to add brood war to this list. The huge campaign, with the addition of those key units made SC turn into the world wide super hit that changed gaming for ever. It's kinda hard not to put this one at the top.
For me,
Durlag's tower in BG was one that took that game to total next level. The polish, the spell additions, loot changes, end game bonuses, fixes everywhere. I dunno of a single RPG add on that made so many change every where. Other RPGs add great additions, but none did what they did for BG. The others that came later were often great, but they did not really effect the base game at all. This one changed everything!
Civ V, the 2 DLC really changed the game. Adding the religion aspect to the game was fantastic. I also liked the tourism aspects, the game finally felt complete.
Crusader kings II. No i'm not saying all of it. the DLC i think that mater was "way of life" Which added a lot of RP depth, then the old gods for playable tribes. To me that's all you need. If you want the game a little harder you can add in Reapers and Monks. Those add a bit of a challenge and a few more events, but the other 2 really changed the game. To me you can basically skip anything else.
Starwars Galaxies Jump to light speed. How can you have a star wars game with out space? well these days there are a ton, but back then there was SWG. JTL brought the star's into Star Wars back then it was a game that rivaled X-win verse tie fighter. if you ever played a space battle with 100+ players in x-wings, Ties, gun ships, Bombers ect.. then you know exactly what i'm talking about. I have not played the new Rouge squadron but i dunno if even that can come close to what you could do in SWGJTL
Honorable mention, I'd have to go with the Dragon age Stand alone missions. Liliana's song, and Morrigan's quest were really fun short games. They gave a ton of background to the character just through the game. Lil's story you really learned her personality, and Morrigan's was that huge cliff hanger from the main game wrapped up in a bow. But the real shining star was the huge part II, Awakenings. It was a complete new game, unique story, new characters, fresh perspective. I often felt like you should just make a brand new warden as it felt more like a whole new game/story than any sort of continuation. It was very much like the add on in DAII, the one with felicity day. Just a short fun side story that should basically be called it's own game but here we are, it's just DLC.