I am picky, and cheap! I ponder a long time before I plunk down money on an expansion, so I have only been picking them up when I had a use for them, or when it is unusually convenient.
Supernatural is what sold me on the game, and it is still a big favorite.
This year, when following a nasty virus I had to re-install the game, I also needed to replace it, my disks having gone AWOL. So I picked up the three-pack that bundled Late Night and High-End Loft Stuff with the game, making Late Night my second expansion.
I mostly play Young Adult Sims, and it seemed to offer a lot for them, so University Life was next. Still quite happy with it, though not currently playing the household I made to explore it.
I had been planning to get Ambitions next (and have). Hardly explored it yet, however, as my game has been totally turned upside-down by both spouses in my latest household wanting to adopt a child together. So, very abruptly, I found it expedient to pick up Generations.
That's it so far. We'll see what makes sense to add on from here.