I ranked my favorites in order from one to ten, but there were also clear divisions between certain packs, so I put them into four categories as well.
Favorites:
1. City Living
2. Get Together
3. Get to Work
Second Favorites:
5. Cats and Dogs
6. Get Famous
Third Favorites:
8. Snowy Escape
9. Discover University
10. Eco Lifestyle
11. Island Living
Fourth Favorite:
13. Seasons
I feel like they all have their merits, but my play constantly makes use of the first three. I love everything about City Living: the interesting outfits, the great build/buy items, the festivals, the new world, and all the small bits of play like finding posters in San Myshuno. I use Get Together's club system all the time, so I would have a hard time playing without it. I love the active careers, particularly the scientist one, and the aliens and alien play with Get to Work.
I would be sad playing without Cats and Dogs, because I love cats and dogs lol, and I love Get Famous because it gave us another active career. I don't really need the fame system, but it can be fun at times.
All four of the packs listed in my Third Favorites category are fun and add a lot to my game, but I could still play without them. Snowy Escape might have ranked higher if it weren't so... well, snowy.
Seasons is by far my least favorite, except for the fact that it added the calendar, which is very helpful. Really, I'm very glad I don't have to choose and get to play with all of them!
