Kinda depends on the player, but I agree that Get Together IS a good base starter pack, and for a second expansion, I think Seasons would be my top choice. I might also recommend getting a "bundle" type thing--1 expansion, 1 game pack, 1 stuff pack sort of thing--and then, for the most general player, I'd probably say to get Seasons, Parenthood, and then maybe Tiny Living.
But, as other have said, probably depends on the person I'm talking to--I definitely have friends who I'd be like, no, you need Realm of Magic, Vampires & Paranormal IMMIDIATELY, and others where I'd say Cats & Dogs, Knitting & Toddler Stuff would be the must haves, lol.
