Probably Griezmann or Pepe.
Ronaldo almost certainly wasn't the best player of the tournament. I mean, he was quite good...but he essentially scored three goals in two different games, and in not one of them was his goal a sole, decisive winner, or even one that could've been said to have won the game for his team alone. His goal against Wales was excellent, but Portugal would've probably still won that anyway.
His overall game wasn't that good, and if we're judging his contributions then it'd have to be by goals, and Griezemann outshone him in that regard comfortably. Ronaldo also never once came up against overly challenging opposition (compared to what France or Germany or even Italy faced), and was injured early in the game when he did face a top team at the tournament.
Portugal's Euro victory could arguably be attributed in large part to their defence at times, and due to that if you're giving it to one of their players it probably should be Pepe or the GK. Nani was also very good, to be fair.