Age of Empires II was a 1999 RTS game featuring 13 playable civilizations from the Medieval period. Those 13 civilizations are split between 4 architecture sets and star in a series of roughly 30 scenarios. The Conquerors was released a year later, and added 5 new civilizations, two of which used an entirely new set of architectural assets and had distinct regional flavor, along with another 26 scenarios (generally featuring a wider variety of cultures than in the original). There were quality of life updates regarding micromanagement, ship formations, improved unit pathing and AI, etc. New mechanics like unique technologies were added to the game. Also, there is a gimmick unit called "Furious the Monkey Boy."

Old World Blues is basically a mini campaign within Fallout New Vegas, which your character in the main game can enter at any point in the story they want, and after the mini campaign is completed, players can travel between world maps as they please. This new mini campaign takes place in a new location, adds to the lore of the game, and introduces many new enemies, items, and assets. The story is often silly, but does have something serious to day about the role of science and research in society. One of the characters is an omnicidal toaster.

Mario Kart 8 had two DLC packs. These cost $8 individually, or $12 in a bundle. Altogether, these added 16 race courses, 6 new racers, 10 new Kart parts, pallette swaps for some existing racers. Of note was the use of crossover content with other Nintendo series, which was a novelty in 2014 and 2015, but it was reserved enough to leave the focus on the Marioi franchise. The new tracks added were a mixture of crossover references, complete remakes of classic race courses, and entirely new tracks made for the DLC. The Presentation of these courses is top notch, with the Animal Crossing course standing out for having four different versions based on the four seasons. Did I mention that this is one of the few pieces of video game content to be released early?

Shovel Knight: Specter of Torment was a new free campaign for Shovel Knight owners, featuring an entirely new playable character with unique mechanics going through reimagined versions of the game's levels and cast of foes, complete with a new soundtrack meant to match the game's tragic tone. There is also an area in the hub world where your character can go to brood dramatically.

Sonic & Knuckles is a game first and DLC second, but I'm counting it here due to how it interacted with other Megadrive Sonic games.

I thought the AoE1 expansion The Rise of Rome was better than The Conquerors for AoE2.

The already mentioned Brood War would be my pick. It is so coneccted with the original StarCraft that when someone mentions StarCraft they pretty much always mean the full package. Same goes for Lord of Destruction for Diablo 2 and probably even The Frozen throne for Warcraft 3. All from Blizzard at a time when they dropped bomb after bomb of masterpieces onto the gaming world.

What is considered an expansion? is something like the character packs fot Smash ultimate considered expansion packs? Or does it need to be more substantial in the story sense of the game?

Pity I don't have a laptop atm. And yup, I suspect your love for the game is the time and the people. 

Strange I never heard of it, I recall Blue Shift, not that I played it but not opposing force at all. 

I agree, Frozen throne and Starcraft DLC are definitely up there. 

Nah, I don't think packs are expansions, they are add ons, much like TW3 done a load of free side quests after release but they were small. A level DLC is different than an expansion but since we are talking about 90's games expansions it's so hard to define cause they were more about recreating that game and adding to it. I can't tell if something like war of the chosen which recreates the game with added content is easily said to be an expansion. The line is so blurry between games.