Be it a certain character, spell, mechanic, ending, etc., Not the expansion as a whole

Reaper : The Reaper

Bloodmoon : Lunar Events

Frostmarch : New Warlock Quests, I love Warlock quests!

Sacred pool : New Quest Rewards, I love them too!

Fireland : Djinn Sultan, probably the strongest card in the game

Harbinger : Descriptions of Omens, they are very immersive and you can feel the effects in-game

Dungeon : Leaving the Treasure Chamber after the Lord of Darkness (subtracting attack scores)

Highland : Sprite, Lost City and Random Relics

City : Tavern Maid, a comical character

Woodland : Everything, my favourite expansion

Dragon : I love the concept art of the 3 draconic lords

Cataclysm : References between boards

Nether Realm : I'm just scared everytime...

Prefer the chronological order: (also, you missed one Expansion)

Reaper: Warlock Quest Cards + the huge number of Adventure cards that it adds to the base game

Dungeon : Dark Denizens, Sister of Fate, having a place where you go to "Level up"

Frostmarch: Warlock Quests Alternative Ending, Transmutor, Fate Bound, Eastern Dragon

Highland: Sprite, having a place to go adventuring that is not as dangerous as the Dungeon

Sacred pool: Cleric, Quest Reward cards, Neutral Alignment Cards, Vindication + Misfortune, Theme and how
it is represented on the Adventure cards

Dragon: Dragon Priestess, Conjurer, lots of Dragons

Bloodmoon: Time Card and how it works with Adventure cards

City: Cat Burglar, Tavern Maid, Jail, some City Events, having a place where tons of gold mean more

Nether Realm: Nether Bane, Changeling, Titan Wraith

Firelands: Terrain cards, Ifrit Sultana

Woodland: Woodland, Path and Destiny cards

Deep Realms: Connection between two Corner-Regions, the "Mini-Bosses" stealing stuff from the Treasures
and the City cards

Harbinger: Terrain cards, Spells, Ascendant Divine, Implementation of the Harbinger NPC

Cataclysm: the Board and all the new cards

Prefer the chronological order: (also, you missed one Expansion)

I know... Deep Realms are sold out in my country I'm searching for it