There are good sales frequently so maybe be willing (if you can) to wait a bit.
I like them all and this isn't my personal ranking of how I like them but for single sims that mostly spend their time away from home some thoughts:
Snowy Escape for the outdoor sports, and hiking, and just the beauty of it. Also of course festivals. Makes for a wonderful vacation world or active sim world.
Get Together for party, dancing and djing (also has a beautiful world).
City Living for visiting areas, art, basketball, festivals and singing karaoke. I don't have my sims live in the apartments usually but I do enjoy this pack. My sims rarely miss the Romance or Spice festivals.
University - Personally my sims only went to classes then home to cram and do homework to get those A's... if you are willing to take them through it slow or don't mind some less that A grades it can be a lot more social. Still ranking it last for you.
Cottage Living is my dream pack.. but for me it's really home and community based so not sure that it relates to how you play as much. I guess it depends. It's a great pack but my sims are all homebodies growing things, canning and taking care of their animals.
Parenthood... not sure this will offer much for single sims that are mostly out of the house either.
Tiny Living same as above although of course all packs offer more to build with and do.
