What do you feel are the best expansions to video games that you have played, and what specifically makes your pick worthy of the title in your opinion?

I'm sure a lot of people are gonna say Witcher 3. I dunno if you could call this an expansion or just a bundle/season pass, but I was really pleasantly surprised by the quality of dlc for RE7. Variety, effort, creativity, it had it all. I especially enjoyed the bedroom and card game episodes.

I've not played many expansions. Usually by the time they come out I've beaten the game and have moved on. The Skyrim expansions Dawnguard and Dragonborn I thought were good. Especially Dawnguard. The Borderlands 2 bunkers and badasses DLC was really good and was the inspiration for Tiny Tina's Wonderlands. 

I don't know if it counts, but the American Nightmare expansion or DLC for Alan Wake was a lot of fun. Yes, less story and more arcade action, but I really enjoyed it.

Ruins of Kunark. It was the first Everquest expansion which improved the game in every way.
Dungeons of Norrath is my second favorite for adding procedurally generated dungeons with a whole new upgrade path to pursue.

For non MMORPG it has to be Baldur's Gate: Tales of the Sword Coast.

Brood War.

That is a good one. I would think it counts, even if it is a standalone expansion. 

Dont really buy much DLC, but Fallout 3 had some good ones.

Probably Torna: The Golden County (XC2).

I'd say Broken Steel (Fallout 3), but that was more to fix the shitty endgame of the core game.

Ff14 heavensward and shadowbringers expansions