#2 Kylian Mbappe = £158,000,000 (Monaco to Paris Saint-Germain)

Kylian Mbappe is a 20-year-old centre-forward who plays for Paris Saint-Germain in Ligue 1. He has also featured 30 times for France since making his debut in March 2017; he won the World Cup with them last summer.
PSG went about signing Mbappe in a very unique way. They brought him in on a season-long loan with an option to make the deal permanent for 180m euros (£165.7m). The delay in signing the striker, who was 18 at the time, was so that they could still comply with the Financial Fair Play regulations after having spent a huge sum of money already that summer.
It was a very clever way of getting round the rules that threatened to stop them from getting him, and is perhaps a method that UEFA should look into soon to prevent clubs from finding more loop holes in their rules.