Pretty cliche, but The Witcher 3.

Rimworld or Crusader Kings. Their DLC expands the possibilities of what can happen during a game and are entirely optional. Their existence doesn’t take anything from new players who only own the base game and you can pick and choose what you want to add to your game to expand it how you so choose.

Rimworld

Enemy Within for XCOM Enemy Unkown was a perfect addon. New storyline, new surprises and more polish to the main game. Well worth the money

No personal experience but Euro Truck Simulator seems to be doing it well

Bioshock Infinite Burial at sea episodes 1 and 2 are the only DLC I ever bought. They close a wonderful history.

Are we counting old-school expansions as DLC? If so, then, aside from the infamous Horse Armor, the Elder Scrolls series seriously raised the bar for what to expect from RPG add-ons. Tribunal and Bloodmoon were massive expansions that set the standard early on.

Knights of the Nine might’ve been a bit weaker, but Shivering Isles is one of the GOAT expansions and is arguably better than the base game.

Skyrim kept the momentum going with Dawnguard and Dragonborn, both of which added tons of new content.

The series is straight-up GOATed when it comes to expansions that are actually expansive: new locations packed with quests, items, monsters, spells, etc. They take already huge games and somehow make them even bigger.

The Destiny and Destiny 2 expansions were great... right up until the one that removed 1/2 the game.

Mass Effect trilogy had some pretty good DLC. Citadel is probably in my top 3 DLC of any game of all time.

Factorio.

Fallout 3 and NV.

The case of the golden idol and the sequel (rise of) both had brilliant DLC which really expanded the lore of the game

Fallout is pretty good. I appreciate that they have DLC directly from Bethesda, as well as the openly approved mods and the ability and openess to allow bigger mods that expand the game. There's a lot of reasonable hate for Bethesda and their timeframe, but they're pretty good on allowing mods that are essentially amateur DLC. I just wish we had an update to really make the series new.

Tie Fighter had some great DLC back when it was just called an “expansion pack”.

Hyrule Warriors Legends and Mario Kart 8 had DLC that doubled the content and didn't cost that much. I probably played more DLC then the main game for both.

Bo2 zombies DLC Was great.

The following DLC From dying light was great, new area, new mechanic (driving) and expanded on the story

Project Hel, a DLC for Ghostrunner. The base game was already pretty great, but the DLC added jump trajectories, making movement less ambiguous and improved the frankly wonky upgrade system of the original. It also added a new (albeit shorter) story, a new rage mechanic and you get to play as a cold, unfeeling cybernetic abomination controlled by the villain of the base game.

All of this is to say that I was floored on how much I preferred playing the DLC than the original and I loved Ghostrunner.

I generally don't like DLC, and even disable them if possible, but Dark Souls 2 had some excellent DLC areas.

Tiny Tina's Assault on Dragon Keep as part of Borderlands 2 was so fun at the time - all of the BL2 DLC was good honestly. Haven't played it in years though.

Technically Far Cry: Blood Dragon was a standalone expansion rather than DLC for 3, but it blew every other far cry game out of the water.

path of exile - no one comes close either - free too

Red Dead Redemption's Undead Nightmare was excellent.

I quite like most of the The Binding of Isaac DLC (although sometimes they're a bit too big, to be honest).

The Vietnam DLC for Battlefield: Bad Company 2 was also rather good. I didn't get to play as much of it as I might have liked but I remember it being more than just a resin.

Bonus: Red Alert 2: Yuri's Revenge (an expansion pack rather than DLC admittedly)