If I had to pick one, little hesitation:

Witcher 3: Blood and Wine - Beauclair is my favourite city in any game ever (Well, maybe apart from Solaris in Xenogears) and Toussaint is my favourite area of The Witcher. It’s a joy to walk around, stunningly beautiful, and while smaller than Novigrad (not by much) is more detailed and with less repetition. I love the heavier amount of dialogue than the previous parts of the game. Vampires and wine. I love owning my own wine barony. I also like how the expansion wraps up more cleanly than the base game, even allowing you to invite Triss or Yen. Blood and Wine is also really big, quite a lot bigger than Torna: the Golden Country, for example, with WAY more dialogue and production… and this is coming from a die-hard Monolithsoft fan. Witcher 3 is a massive game on its own, and Blood and Wine is like a beautiful dessert at the end. And there are some real gems in the side quests: Roach talking, the bank bureaucracy, and getting your own wine. I also like all the stuff with knights and chivalry.


Other’s I’d like to mention:
Witcher 3: Hearts of Stone - while Blood and Wine is my preference (being it’s like 8-10 times bigger) Hearts of Stone has IMO hands down some of the most memorable sequences of any game ever: the wedding, Gaunter, and Iris’s tragedy… also, you have a heart of stone if you don’t like Shani… OK, she’s a Mary Sue, but that’s fine for side characters. The expanded Redania regions are also quite nice too, only to be topped by Toussaint in Blood and Wine.

Crusader Kings 2: The Old Gods - countless hundreds of hours playing as Vikings :)

Star Craft: Brood War - would probably be second, this game remains to this day the online game I dumped the most amount of hours into. I haven’t found any other online game I like all that much, but I love this one. I played this much more than the base game.

Sid Meier’s Civilization 2: Fantastic Worlds - added in a bunch of features including macros. I got really heavily into modding with this game as a teenager. Also, one of my first expansions ever.

Warcraft 2 expansion (forget the name) - nothing special, but for a young me, I spent a lot of time on this. I forget exactly why, only that I really liked the game. It was one of my first expansion packs ever.

You guys are getting me excited for the Witcher 3 remaster, if that includes the DLC that is. It's still slated to come out before the end of the year? Q4 2022 is the latest info I could find. I never played the DLC and played Witcher 3 when it came out with all the warts, bugs and glitches of the release version on ps4. Playing a finished polished game would be a nice welcome. Same for Horizon Zero Dawn remaster with the Frozen Wilds included.

Mysteries of the Sith

Though it's stand-alone so I'm not sure it counts.

Mario Kart 8, best DLC/expansion ever IMHO.

For me, it's gotta be Monster Hunter Rise Sunbreak.

The base game was my GOTY for 2021, and the expansion retains everything that made it great while upping the challenge to make those hard-fought victories feel all the sweeter.

The challenge level is tough without feeling unfair, the new maps and monsters are awesome, and thanks to new mechanics and tweaks it plays better than ever. It's geared for enthusiasts, but it's the most fun I've had a game this year.

For me it has to be Half-Life: Opposing Force. It's a fantastic few hour experience and my favourite Half-Life game.

While StarCraft: Brood War and Diablo II: Lord of Destruction are my two favorite expansions ever, I'd also include Diablo III: Reaper of Souls in this list. I'd say RoS didn't raise DIII to the heights BW and LoD did for their respective games but it did turn the horrible mess that was Diablo III into an actually enjoyable game that I spent a few hundred hours playing.

The Last of Us Left Behind

Uncharted The Lost Legacy

Ada's Campaign added for RE4 on PS2 and beyond

Witcher 3's Blood and Wine and Xenoblade 2's Torna the Golden Country deserve a mention too for me, both feel like fleshed out adventures in their own right with more content than many whole games.

Most recent:

-Outer Wilds: Echoes of the Eye

Given the main game feels complete, you'd think they were riding on its coattails. Instead, its contrasts compliment the rest incredibly well.

Overall:

-WoW: Burning Crusade
-WoW: Wrath of the Lich King
-The Last of Us: Left Behind
-Outlast: Whistleblower