Gakpo. I think that after Jota (RIP), he has the best finishing rate of the attackers (haven't looked up the data to confirm, but it's a feeling I have).
He's actually marginally outscored Diaz across all competitions in each of these 2 seasons (16 and 18 versus 13 and 17). I looked at the Player of the Season votes here, and there are 20 points separating the two of them in Diaz's favour.
Now, I don't think Diaz is overrated, in fact, I agree with others here that he seems to be underappreciated. Gakpo is less flashy and also less showered with plaudits, but I think he's more efficient and lethal.