| The only one I really don't care for is the princess & dragon. The dragon's rarely effective and the fairy is just a nightmare to keep track of. Plus, the magic portals screw with one of the fundamental bits of Carcassonne (that you can't put a dude on a tile that's already been laid)
A lot of people could take or leave the tower. I think it's okay.
Inns & Cathedrals is pretty much a must-have, if nothing else for the 6th set of pieces, but the big meeples add a lot of interactivity to the game. The eponymous inns and cathedrals are also nice, though don't really add a whole lot.
Builders & Traders is another popular one, though it adds a lot to the game and moves a bit further from the simplicity of normal Carcassonne. May be a good or bad thing, depending. Personally, I can never remember what the pig does!
Haven't tried the Abbey & Mayor.
I like the River and River II combined.
I've only used the Count of Carcasonne once, and don't recall being impressed. My biggest complaints were that the art style was slightly different from normal carc, the setup of the tiles mattered, and most egregious of all, it meant not using the river! | 
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