My essential pack is actually Vampires. Vampires for me was like the Toddler drop for family players. I love everything about that pack and there is amazing balance to the Vampires because they have to have a weakness for every few powers they get. They are probably one of the darkest features in TS4 (even though they got gimped from feeding on sleeping Sims) and the gorgeous, grungy Victorian aesthetic and gothy, edgy CAS just pushes it to perfection. Most other packs I would personally recommend are also GPs. I just get a ton of enjoyment out of them.
If I had to choose an EP though... I don't know, it's hard. I love Island Living because the world is just gorgeous and nice to "be there" and has a relaxing atmosphere and Mermaids, which I love a lot more than I expected to. But it feels small, so I'd recommend grabbing it on sale only, if you're not into the game in the first place. But it does have a few fun water activities. I like the more subtle features of the pack, though.
Get Together I like for the large world of Windenburg as well as the club system and a little more spice to night life. The club system is one of those features that can be overlooked and underestimated but you can actually set up some group behaviors with it and choose hangout locations so if you like to switch households, you'll actually see those Sims show up in the places you set, doing the club activities you chose (as long as the right requirements are met at the locale). Lots of creative things can be done with clubs and using townies or extra Sims to bring areas more to life. You can even set up rival clubs who like to fight and have them hang out at the same place for some drama, lol. They can also be used for family gatherings or senior clubs or age groups. It's pretty versatile.
City Living is actually pretty good too for the world itself and apartments, and I didn't like it when it first came out. I love the apartments, despite not being able to build or move them. And I believe a bunch of lot traits came with this pack. Lot traits can also mix things up a little by adding behaviors to locations or homes. They can add benefits or detriments, depending what you want. I think lot traits are a great additional feature.
