I haven't played any of the spin-off/variations but I have played most of the expansions to the base Carcassonne. I have the "Big Box" edition, which comes with Inns & Cathedrals, Traders & Builders, Princess & The Dragon, Towers, and the River. I also have The River II and "The Count of Carcassonne" (haven't played with the Count).

I like that the base game is relatively short, particularly since our other board game of choice, Settlers of Catan, can run a little long. Inns & Cathedrals doesn't add that many tiles and only adds large Meeple so I don't think it extends the game by more than 15 minutes but makes town and road building more interesting. My friends and I used to practically ignore roads before this expansion, instead focusing on farm domination. ;)

By far the best expansion is Traders & Builders, in my opinion and the opinion of my friends. It makes the game a bit longer but encourages people to finish off other people's cities and adds a new dimension to the game with the trade goods.

I haven't played Towers or Princess/Dragon much because they make the game MUCH more competitive/aggressive, which results in some sore losers. ;) I think due to the extra strategy involved they make the game much longer than the other expansions despite having fewer tiles.

I highly recommend "The River II" - I like it better than the first River. It really helps to chop up the farms at the start of the game to give players a more even chance of starting with a decent farm - especially if you're playing with more than two people.