I love prosperity. At first, I was playing with a friend who had all the expansions available at the time (before Hinterlands, Cornucopia and DA), and then I bought the original, intrigue and seaside. I realized all too quickly that all my favorite cards were in Prosperity.
I got it as soon as I could
Hinterlands and Cornucopia have added a lot of fun cards too, and after playing with them on iso, I have come to enjoy them as well, but my most favorite cards still are mostly from Prosperity. Chances are, if I name a card I like, without thinking what set it comes from, about half will be from Prosperity. Maybe its just me.
I recently got Dark Ages, and that's been fun, but I don't feel familiar enough with the cards to say if its my favorite or not. There's a lot of really cool stuff going on in there, I definitely love playing with looters and spoil gaining cards, but I haven't figured them out well enough to have favorites yet.