Best Imperial Assault Expansion?

My friends and I are very close to finishing our first campaign of the base game of Imperial Assault and we absolutely love it. I know that we'll want to keep playing right when we finish so I'm going to go buy one of the expansions soon. I just wanted to know if anyone liked any of them in particular or if there were some duds. Otherwise I'm just going to go in order of release and buy Twin Shadows.