I have all EPs except Showtime and Seasons, no stuff packs. I do have a fair amount of store content though.
I might get Showtime at some point, mainly for genies, the pool table and that gyroscopic spinning thing that seems really cool. The professions seem like they're a nice option, but I don't like the idea of my singer spontaneously bursting into flames. I suppose I could save that career until I travel somewhere and get a bunch of showers in a can. For similar reasons (among others) Seasons hasn't grabbed my attention so much, even though there are a lot of other features that do appeal to me.
University Life is on my radar, though I'm going to wait for a sale like I did with most of the other EPs except for Supernatural which I pre-ordered.
Of all the EPs, Ambitions is the one that I couldn't live without, followed closely by Supernatural. Pets and World Adventures are equal to me, albeit for different reasons, though to be fair I haven't had Pets for very long. Generations is the one I probably use the least, however the motive mobile is a beautiful, beautiful, beautiful thing to have, and when I do have minors in the game it's nice that they have more things to do than they would otherwise...also, boarding school!
Late Night can get irritating with the celebrity system and Bridgeport is a pain with all the skyscrapers (to the point where I just don't play there), but I enjoy it more now that I can turn off celebrities at my discretion, and I never uninstalled it because I use the clothing and objects from that EP more than any other (although Supernatural is catching up).