I've never understood the fascination with Cities & Knights. IIRC, that's the one where early in the game the pirates come and whoever is in last place loses a settlement. That just seemed awfully raw in a game where getting a slow start is already a double whammy because you're behind everyone in points and production.
Seafarers is okay, especially the way it rebalances the value of sheep, but my favorite spinoff is Starship Catan, the two player card game based on Starfarers of Catan. Much better than the Catan Card Game, IMHO.
I never played the Stone Age one. That came back after I'd tried to rein in my game spending, and at that point I figured I had plenty of Catan related stuff.

I'm partial to Cities & Knights and, in fact, find it difficult to get back into regular settlers now. It's a great game, especially when combined with Seafarers. I've also played the Settlers of the Stone Age game which I enjoyed quite a bit (I ended up giving it away though when I moved from California to minimize car space).