That is difficult question. It depends very much how experienced player you are in Sims 4 and do you have any pack already, but my starter bundle would be:
EP: Seasons, it just add so much depth and feel into the game. It has one of the best build/buy items and good cas too.
GP: Parenthood, spa day or even dine out. Those two are like not so specific packs and add generally much. You don't play so much with outdoor places etc.
SP: Backyard stuff probably. It has nice build/buy mode and it adds nicely depth with seasons especially in summer.