3. Neymar - Barcelona to PSG, 2017 (€222 million)

If Paul Pogba's move from Juventus to Manchester United inflated the transfer market, Neymar's move from Barcelona to Paris Saint-Germain blew it wide open. This move away from one of the biggest clubs in the world to the Ligue 1 champions is historic and shocking for several reasons.
Firstly, Barcelona was one of Neymar's dream clubs growing up in Brazil. The forward won the treble with the Catalan club and formed a deadly partnership with Lionel Messi and Luis Suarez. No one expected the Brazilian to leave, but apparently, he wanted to move away from the massive shadow that Messi cast. At the time of his departure, Neymar was the third-best player in the world, and his move changed Barcelona and PSG.
The former has not won the Champions League since Neymar left, whereas the latter might go all the way soon if the Brazilian can find his old form again. Secondly (and more importantly), the massive amount of €222 million which PSG paid for Neymar's services broke the transfer market.
The Paris based club set a precedent that unnatural amounts of cash have to be thrown around to buy the best of the best, and that trend has continued. Since Neymar's move, the likes of Kylian Mbappe and Joao Felix (who were teenagers at the time of their transfer) have been sold for over €150 million.
Goalkeeper such as Alisson Becker and defenders such as Harry Maguire have also been sold for amounts which would have been as ridiculous not five years ago. Due to these reasons, Neymar's inclusion here is warranted.