Which is the best sims 3 expansion pack?

well it depends on what you want for gameplay that either one will add to your game play here's an example of four  expansion packs

late night

become a vampire and learn skills quicker than a normal sim ,raid the hospital or grocery for plasma)( although you will only to drink plasma juice from the fridge vegitarian vampires can eat plasma fruit which is bought from the grocery) can grow plasma fruits

turn others into a vampire, place discos .bars clubs into your other worlds and not just the home world of bridgeport ,<--new world to play in play piano,drums or bass form a band , new clothes

uni

no new home world  just a subhood your sims if you want them to go to university adds new traits new objects,new clothes  updates your phone to a smart phone

new skills , play table tennis if your sim does go to uni to get a degree they will get a boost in their chosen carrer i.e. start at a higher level than a sim that does not have a degree adds new plants to grow if you like gardening

you do not have to send your sims to uni either  for the new objects as those can be used in your home world

pets  teach dogs tricks - play with cats, dogs and horses - train horses and win races adopt a pet appaloosa plains new world that comes with pets

 edit typo