Bloodborne The Old Hunters is the single best expansion I've ever played. It has the best bosses, and the base game already had fantastic bosses.
Dark Soul 3 Ringed city.
DA Inquisition - Tresspaser. I guess Jaws of Hakkon and The Descent were fine as well.
Counting MMORPGs, I guess Burning Crusade from WOW. It was the best WOW has ever been.

Other than that, I guess the usual answers, Witcher 3 Blood and Wine and Hearts of Stone, Skyrim Dawnguard/Dragonborn, Horizon Frozen Wild. Theres a reason they are the common answers, because all of them are great.

Sorry to be typical but it’s just the truth - Hearts of Stone and Blood and Wine. The Witcher 3 already blew me away with the base game; what I found with the expansions really catapulted that game and experience into the stratosphere for me, and the ending of Blood and Wine really made me feel like a truly epic adventure had come to a wholesome, natural and satisfying conclusion. I still get chills thinking about how it all went even now, and even feel a bit emotional! (Ok, a lot). I can’t say if the game would have been my favourite of all time without the expansions but with them, it dethroned my two decade long favourite game ever, Majora’s Mask. A truly staggering achievement from CD Projekt Red.

Don't play many expansions/DLC but I think these should generally rank high in most people's list if they played them:

ME3: Citadel
HorizonZD: Frozen Wilds

Some of my favorites:
-Both of Witcher 3's expansions
-Borderlands 2's Tiny Tina's Assault on Dragon Keep DLC
-Fallout 3's DLC's

I never play DLC anymore since it comes out after I've put the game behind me. But this does make me interested in the remaster since those include the extras. Same for Witcher 3, I would buy a PS5 remake with DLC included.

1) StarCraft: Brood Wars.
2) Diablo 2: Lord of Destruction.
3) Alpha Centauri: Alien Crossfire.
4) Elder Scrolls Oblivion: Shivering Isles.
5) Baldur’s Gate II: Throne of Bhaal .
6) Dragon Age: Awakening.
7) Neverwinter Nights: Shadows of Undrentide and Hordes of the Underdark.
8) Dark Reign: Rise of the Shadowhand.
9) Total Annihilation: The Core Contingency.
10) Dungeon Keeper: Deeper Dungeons.

I feel like expansions haven't really been notable in recent years, or my tastes lay with older games.

Ones so important they're basically integral to the game now:

• StarCraft: Brood War
• Diablo II: Lord of Destruction

Great DLC, though not essential:

• Mass Effect 2: Lair of the Shadow Broker
• Bloodborne: The Old Hunters (mostly because the lead up to and then the fight with Orphan of Kos is amazing)
• Horizon Zero Dawn: The Frozen Wilds
• Fallout 3: Mothership Zeta

I will always have a soft spot for Warcraft II: Beyond the Dark Portal.

Yeah, that's some weird BS, they had their own launchers and menus as well. Beyond the Sword actually had a couple of good mods as well. One of the best conversion mods for any game I ever played was Fall from Heaven II on Beyond the Sword, it's a fantasy mod with amazing new mechanics and factions.

Bloodborne: The Old Hunters - Some epic fights. Ludwig is probably the toughest fight overall, more than any in the base game.
Torna: The Golden Country - I love the atmosphere and it had a lot to do.
If we're talking about more peaceful/less advertised (I guess) I would say
Welcome Amiibo for New Leaf: A lot of new and cool things.
2.0 Update for New Horizons: So much added in a lot of the updates.
Stardew Valley updates