Great idea! I'm always interested to see what people like about the packs I don't own yet. I'll only give opinions on the packs I have. :)
Expansion Packs:
1.Get Together
2.City Living
3.Cats&Dogs
4.Get To Work
Why is Get Together my favorite? Because the addition of clubs is useful in countless situations. Regular club activities aside, I use the club feature to give bonuses to legacy kids building skills, gather "employees" to tend gardens or bake for a retail shop, and conduct birthday parties and weddings with much more control than the party system.
Why is Get To Work my least favorite? I LOVED Open For Business in TS3, but in comparison I find Get To Work has a massive imbalance between investment and payoff. I find it discouraging that I could spend 3 hours talking to a customer to sell an $8 cupcake, or a $3000 painting. It feels pointless to try to run a bakery when it takes the same amount of work and you get much less payoff. Don't get me wrong, I still love it, but running a retail store doesn't fit into every game or life style as much as the other expansions do.
Game Packs:
1. Parenthood
2. Outdoor Retreat
3. Spa Day
Why is Parenthood my favorite? I love playing legacies, and this game pack adds an incredible new level of interaction between siblings and challenge in raising children. It also enhances the leap ahead a sim can gain if they are groomed from a young age instead of created.
Why is Spa Day my least favorite? As the most recent pack I've purchased, I haven't been able to explore it fully yet. I'm not a huge spa person myself so I don't think my Sims will visit it enough to make the pack worth it's full price, but I love all the build mode items in this pack and the relaxing feel it brings to the game.
Stuff Packs:
1. Toddler Stuff
2. Holiday Stuff (free)
Why is Toddler Stuff the only stuff pack I actually own?! I find it difficult to justify paying just for "stuff" but as I mentioned above, I'm a huge legacy player and the base game is severely lacking in content for kids. I feel silly for giving EA money purely because they failed to add more variety in the first place, but at the same time I can understand since toddlers were added late.
Why is Holiday Stuff a favorite? I wouldn't likely purchase this pack if it had a price, but I do enjoy the additions in order to stage holiday chapters and events in my games. The gift tower alone is useful in so many situations!