Stand alone DLC isn't counted so stuff like Imfamous first light and Uncharted Lost Legacy are out. It has to be an expansion of the game that's tied to the base the game in some form or another through either DLC or a CD expansion of the 90's on PC. I'm gonna list my top 10 but number 1 is by far...

1. Blood & Wine - The Witcher 3

2. Hearts of Stone - The Witcher 3

3. The Old Hunters - Bloodborne

4. Iceborne - Monster Hunter World

5. War of the chosen - Xcom 2

6. Heavensward - FF14 online

7. Shadow Bringers - FF14 online

8. Dragonborn - Skyrim

9. Iki Island - Ghost of Tsushima

10. Shadow of the Erd tree - Elden Ring

Honourable mentions go to Tales of Arise Beyond the Dawn which I have yet to boot up but I know for a fact it'll be close if it doesn't make it into the top 10. Looks right up there with the best. 

Hmm my top 10 would probably be

1: StarCraft: Brood War

2: Warcraft 3: The Frozen Throne

3: World of Warcraft: Wrath of the Lich King

4: Cyberpunk 2077: Phantom Liberty

5: Gears of War 3: RAAM’s Shadow

6: The Elder Scrolls IV Oblivion: The Shivering Isles

7. Destiny: The Taken King

8. Diablo II: Lord of Destruction 

9: The Witcher 3: Blood & Wine

10. Sid Meier’s Alpha Centauri: Alien Crossfire

Torna Xenoblade 2

MK8

Wolfenstien The Old Blood

1) StarCraft: Brood Wars.
2) Diablo 2: Lord of Destruction.
3) Alpha Centauri: Alien Crossfire.
4) Mass Effect 3 - Citadel.
5) Dragon Age Origins: Awakening.
6) Dragon Age Inquisition: Trespasser.
7) Elder Scrolls Oblivion: Shivering Isles.
8) Morrowind: Tribunal and Blood Moon.
9) The Witcher 3: Blood and Wine.
10) Command and Conquer: Red Alert 2 - Yuri's Revenge.

Mmm I don't exactly have a Top 10 since most of the games I play don't usually have a DLC expansion of such scale.
Well except for :

Xenoblade Chronicles 2 : Torna the Golden Country
Xenoblade Chronicles 3 : Future Redeemed

Mario Kart 8 Deluxe Booster Course Pass is probably among them due to the sheer volume too.

Half-Life: Opposing Force comes to mind. It's not the biggest expansion ever, but I did enjoy it quite a bit back in the day.

Other than that, I didn't play too much expansions, I'm afraid. At least nothing got stuck in my memory too much. I got all expansions for ETS2, but those are basically just map extensions. I also remember enjoying expansions for The Sims and The Sims 2 a lot. Oh, and of course expaniosns for Civilization games.

1- Bloodborne the old hunters

2 - MGW Iceborne

3 - Both Witcher 3 expansions

4- Elden Ring SOTE

5 - The rest

Monster Hunter Rise: Sunbreak

Retains everything that was awesome about the base game while adding a ton of new content and a level of challenge that's perfectly tuned to be exhilarating but never frustrating.

The new monsters and maps are fantastic, the tweaks to gameplay make it even more fun, and there's more to sink your teeth into in terms of playtime than most entire games.

You have exceptional taste, sir. 👏 

Yep, Sunbreak is actually bigger than the base game was at launch and of you play Sunbreak right after beating Rise you get the end game content without grind. My big problem with it, is it gets just too damn difficult, I don't think it's perfectly tuned like you say, it's perfectly tuned for MP/NPC fights but getting through it solo is a pain in the ass, it would likely rank higher on my list if I could get past MR 4. I liked that you could solo basically everything in Rise without attrition based battles... but I suppose it was the same in Iceborne, so maybe my skills are declining.