The Nightlife and university expansion packs for Sims 2 were pretty much vital to have a trully whole game. Since nightlife adds so many core gameplay elements and university adds an entire life stage filled with many core gameplay elements. The others just added arbitrary elements as far as I know.
What a coincidence that those are the two expansions that came out first...
In order of must buy expansions for the Sims 2 I would go as follows
-Seasons: Though it has its fair share of drawbacks it is by far the most fleshed out and well put together expansion of them all. Seasons, Extra jobs, Extra activities, planting, and all that
-Nightlife: This is the best complimentary expansion you can get and includes an extra neighbourhood as well as great ways to collect gold aspiration
-Open For Buisness: Alright the Buisness side of things is pretty tricky however who is to say this expansion doesn't have more perks? The Best supernatural Sim, extra things to do around the house, clothes.
-University: The university aspect of the game is either too short or too long but it does have a few more objects.
-Bon Voyage: Tied with University... the major problem with this expansion is that it is full of legal cheating that makes all the other expansions look rather fair.
-Freetime: OHH MY GOSH!!! This must be the worst expansion of them all... Really beyond Lifetime aspiration rewards (Yes there are two kinds of them... this game featuring the second kind) this game offers nothing else.
Some get special mention are
-Pets: Pets is an expansion no one can dictate when you should get it... If you could care less about pets... Skip it... If you must have pets... Then get it! This expansion can actually make the game less enjoyable for those who dislike pets.
-Appartment Life: I don't know it well enough to really judge.