It's a tie between GP and EP. When I first got sims 4 I was completely focused on EP because that was the format of DLC I was used to but now I think GP are equally good in terms of quality and interesting themes.
The packs I like most, the ones I can't play without anymore are a mixture of EP and GP. My favourites are realm of magic, seasons, get together, jungle adventure, strangerville and get to work. So a little bit of both categories.
EPs have the advantage of a broad contant that is more like to add to everyday gameplay. (perhaps with the exception of island living and snowy escape perhaps) The strong point of GPs is the narrow focus on one theme which allows enough depth for said theme to matter.
SP are fun too but not essential. The only exception is laundry day because it adds a gameplay system to be seen in my sims everyday life.
