| Hmmm, controversial but both Cities & Seafarers of Catan were good expansions to me. Ditto on the Expansion Cards. I'm not sure of Traders & Barbarians (I haven't played it yet).
Carcassonne - the Inns & Cathedrals expansion was good.
Descent - Well of Darkness and Road to Legend are both great. Well to fix some of the problems and the introduction of treachery cards. Road 'cause of the campaign system.
WoW the Board Game Shadows of War - fixed a lot of issues of the main game. Still doesn't make the game great (still too slow), but it fixed a lot of issues. So as an expansion, great! | 
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