#4 Jan Oblak (£75,000-a-week)
Oblak is widely considered as one of the world's best goalkeepers at present, with only Manchester United's David de Gea held in higher esteem. The Slovenia international has been brilliant for La Liga side Atlético Madrid over the past two seasons and continues to impress with solid goalkeeping displays when called upon.
He has won the Ricardo Zamora Trophy for La Liga's best goalkeeper in successive seasons during the 2015/16 and 2016/17 campaigns.
Currently on a £75,000-a-week contract under Diego Simeone's management, his deal runs until the summer of 2021 and there is a hefty €100m release clause inserted within the contract to scare away interested suitors.
Despite this, there are clubs across Europe who would be willing to double his wages should they persuade him to look elsewhere. He has kept quiet about speculation and at 24, potentially has at least a decade of football left at the highest level.
Simeone's term at Atlético is gradually coming to an end, at which point, Oblak's future could become uncertain.