Haven't played many expansions in general yet. Bowser's Fury takes the cake for me easily.

Agree. It is pretty much vital for the game in such a way that when someone talks about Starcraft one simple assume that Brood war is part of the game.

I would say; Lord of Destruction for Diablo II, Blood and Wine for The Witcher 3, Beyond the Sword for Civilization IV, and Throne of Bhaal for Baldur's Gate II.

Diablo 2: Lord of Destruction
WarCraft III: The Frozen Throne

Technically Wizardry II and III could count, since both need Wizardry I (well, a savegame from that one would do, but hey) to work. But back in those days (early 80's), expansions were still unknown. Heck, they're also among the first sequels in videogame history! So of course how to classify them was an unknown matter at the time.

I forgot the Baldur's Gate expansions. Yeah, those are awesome, too.

With Civiliation IV it always annoyed me that you couldn't play with both the Warlords and Beyond the Sword expansions at the same time even if you got both.

WoW: Burning Crusade.

It was more of WoW Classic, but better. This expansion was pretty much the peak of the MMO genre.

The Old Hunters DLC for Bloodborne probably.

This and Torna: The Golden Country

Bioshock Infinite: Burial at Sea
Kingdom Come Deliverance: Woman's Lot
Frostpunk: The Last Autumn
and both Witcher 3 expansions from the top of my head.

Currently playing Hot Wheels expansion on Forza Horizon 5 and it's pretty cool too.

I like Frozen wilds from Horizon

Also Final Fabtasy XV expansion was cool and helped to improve the game that felt convoluted and unfinished