#1 Matthijs de Ligt (Ajax)

Matthijs de Ligt is a 19-year-old centre-back who currently plays for Ajax Amsterdam in the Eredivisie. He also plays for the Netherlands national team, having made 13 appearances for the first team since his debut back in March 2017.
De Ligt has been making waves across Europe for several years now - despite still only being in his teens. He has been a part of the Ajax first team since the age of 17, which is indicative of just how great a player he already is.
Defenders often reach their peak at 27-28, so the fact he's managed to play as well as he has at such a tender age could mean that by the time he reaches his late-twenties, he'll be one of the best in the world.
De Ligt doesn't have a concrete value in the market at the moment, with figures ranging from £20m to £60m, so it's difficult to know what sort of price Barcelona would be able to snag their man at. As mentioned before, Barcelona have an exorbitant amount of cash available to spend, but it depends on whether Valverde is willing to pay so much for just one player.
One thing is for sure, though; if De Ligt was to join the Barcelona defence alongside Kalidou Koulibaly next season, there'd be no getting past them.