Throne of Bhaal for Baldur's Gate II
Mask of Betrayer for Neverwinter Nights 2
United Offensive for Call of Duty 1

Oh yeah, Iceborne. That's like a sequel more than an expansion. That might be the best. 

I found it more fun than then base game because you were always in a company of bots.

Y’know what? I’m changing my original answer: MK8DX Booster Pass. The two year stream of gradually filling the game with a collection of courses which doubled the overall available content was some of the most fun I’ve ever had with a game expansion. Hearing all the new music, seeing how old courses would be remade to fit into a more modern MarioKart experience, playing through the time trials and grand prix, it was just too good!

OH yeah, every MH expansion feels like a whole new game, theres a ton of new content and gameplay changes. Iceborne is easily my favorite, but the MH4U expansion and Sunbreak in MHRise were great. Really excited about the upcoming MHWilds expansion. 

I'm apprehensive. 

Yup Frozen Throne and Wrath of the Lich King.

I think of I listed a top 10 Blizzard would take half the spots with also The Burning Crusade, Heart of the Swarm and maybe Reaper of Souls or Cataclysm (which was great until the final patch that sucked).

Other than that Cloud and Joker for Smash Bros are the best expansions.

(And of course Witcher 3 ones)

Edit: Actually let's do this:

1. The Frozen Throne
2. Wrath of the Lich King
3. Heart of the Swarm
4. The Burning Crusade
5. Blood and Wine
6. Hearts of Stone
7. Half Life 2: Episode 2
8. Cloud for Smash Bros
9. Joker for Smash Bros
10. The Talos Principle: Road to Gehenna

Honorable mention: Reaper of Souls, Cataclysm, HF2: Episode 1, Dawnguard, Persona 5 Royal (an expansion with added steps, top 10 if counted)

Can't recall many for myself but I did really enjoyed the Lost in Nightmares & Desperate Escape addition for Resident Evil 5 (Gold Edition).

It's Hearts of Stone or blood and wine for me. I can't tell which is better cause HoS is so curated and lean in it's content but BaW is more open and up to yourself to make the best of. It's such a hard choice, by content BaW wins but then you have the wedding quest in HoS and characters that exceed anything in video games before this. Ugh, I cannot choose.

Blood and wine. Vampires are awesome