I play Sims 2 everyday! I love it! I haven't bought any of the expansion packs though. Maybe I'll buy one of them soon. Which one is at the top of your list?
That's a tough call. It depends on what you like about the game.
University added a new life stage, 4 new careers, the cowplant, resurrection phone, zombies and the plastic surgery machine....among many other things. I liked that you could send a teen to college as soon as they turned into a teen. Then there are greek houses, cheerleaders and secret societies.
Nightlife added cars, but now you can get them through the two Ford downloads at thesims2.com. The Downtown area is neat with all the different lots, turn ons and turn offs, dating, group outings and the gypsy matchmaker. Then there are the Divas and vampires andespecially downtownies, who are higher skilled and richer than townies and bring goodies when they join a household. Then there's Mrs. Crumplebottom.....
Open For Business was great in that, sims could own and operate community lots. It can be a lot of work but it's a whole new way to play. Sims can earn business skills and also craft skills like toy-making, cosmetology, flower-arranging and robotics. Sims can build their own Servos, which are really neat. I love how Servos can become vampires with their little fangs.
Pets is my least favorite. I don't own any in real life, so having the ability to recreate them for my sims was fun to try but that's all there was to this pack. Pets can have careers though, which helps with family finances. Having werewolves was meh.
I LOVE Seasons. It permeates all aspects of the game. Weather and seasons are great for holidays and it keeps daily sim from getting too predictable. It also came with a new base neighborhood, which highlighted the addition of gardening. Plantsims are interesting and great if you have to garden. Fishing was also added and when sims cooked the fish and items from their garden, their food would sparkle and would let them go longer between meals. I liked opening home businesses selling all the fish my sims would catch. Having six new careers and some handy career rewards also improved gameplay for me. Each season had benefits, too. In Spring, romantic relationships built faster. In summer, friendships grew stronger faster. In winter, family relationships got boosts. My favorite is Fall because sims build skills faster and that includes pets learning commands and behaviors.
Bon Voyage has been great but don't get it yet. They need to patch some bugs first.
The Stuff Packs are just objects, clothes and hairstyles, so if a theme appeals to you, go for it. I loved Glamour Life Stuff and H&M Fashion stuff. Family Fun stuff was OK. I may pick up Celebrations at some point. I'm thinking about the upcoming Teen Style Stuff.
No matter which pack you pick, if any, make sure your PC can run it. System requirements have been evolving these past 3 years. If you wait a little while longer, you could buy all the franchise packs together like you can do now with the original Sims game.
I hope the patch you guys are talking about will be included in the installation.* I have trouble with my game crashing all the time.* It just "freezes up" then I get a blue screen with a bunch of letters and numbers and stuff like "stuck in an infinite loop."* I have to shut the computer down, restart, and then restart the game.* It's extremely annoying.* I've learned to "save the game" often, so I don't lose so much every time it crashes.* 
