The strongest teams right now are:
Juventus: they retained most of their best players, and they bought Bonucci, Perin, and Mr. Champions League himself. You know who.
Atlético de Madrid: they retained most of their best players, and they bought Lemar, Gelson, Rodri, and Kalinić. And they have one of the best coaches in the world. Plus, they have Griezmann.
Barcelona: A deeper squad is what I think they lacked last season. Valverde is an outstanding coach. And they have Messi.