Expansions or DLC? Assuming just expansions my next two parags are about expansions, while the last is about Civ VI DLC.
I think Civ IV's Beyong the Sword was the best Civ expansion, in part because it just had so much content that deepened the game (with only occasional misfires). It also had the most creative scenarios in part due to community efforts for strong mods like Fall from Heaven (fantasy).
For Civ V, Brave New World was good (added tourism, interesting new civs and wonders, with many civs like the Shoshone, Austria, and Venice having cool memorable abilities other base game civs simply lacked).
For Civ VI DLC I'm fond of the Khmer/Indonesia pack, and not just because those civs are OP--their music is great and the civs are fun to play.