I highly recommend 'Seasons' or 'Get Together' if you're looking for an expansion pack. If you're looking for a game pack, I would normally suggest 'Dine Out'... but there's currently a bug in the game that completely makes that pack unusable at the moment, so my next suggestion would be 'Parenthood' or 'Spa Day'. As for stuff packs, my personal favorites are 'Backyard' and 'Kids Room'.
I do NOT recommend 'Outdoor Retreat' or 'My First Pet' (unless you like the idea of buying DLC for DLC).
But, ultimately, it's up to you.