Not sure if it counts as an expansion, but Kingdom Hearts 2 Final Mix: introduced critical mode difficulty. The best difficulty mode and combat in the KH series which required you to use every tool in Sora's toolkit. Absolute kino.

Monster Hunter World: Iceborne: I know it brought the controversial clutch claw mechanic, but I played gunlance and could just ignore it lol. G-rank difficulty, a bunch of QoL changes, a great roster of endgame monsters, and endless grinding mode, the Guiding Lands.

Monster Hunter Rise: Sunbreak: After the really disappointing and lacking Rise endgame, Sunbreak comes out and brings back the monster hunting in Monster Hunter. Weapon balancing, more wirebug moves, insane armor sets and abilities, armor skill customization and rerolls, fan favorite endgame monsters are back, npc followers can join you on hunts, and endless monster grinding is back. Just a complete 180 on the opinion I had on base game Rise, it saved this generation of Monster Hunter.

World of Warcraft: The Burning Crusade: got a lot of great memories playing this since this was the only expansion I was in a chill raiding guild and we got furthest progression on our server. Horde got paladins and I rerolled to Blood Elf paladin, my favorite class. Played a lot of PvP and did well in arena.