#1 Davy Klassen (Everton) - £23.6 million

Davy Klassen was signed from Dutch side Ajax for £23.6 million after two extremely impressive seasons. Everton thought that they had landed one of the top talents from the Dutch Eredivisie, but like many other transfers from the Netherlands, Klassen is a transfer that has somehow failed to pay off.
Being the captain of Ajax and after being signed for such a high fee, it's extremely surprising that he just made 7 Premier League appearances, totalling just 250 minutes throughout the season. However, the fact was that he failed to impress in any of those matches he played in and neither contributed a goal or an assist despite playing in his favored position behind the striker.
Everton gained nothing from the cash they splurged on Klassen and his signing has been one of numerous other failed deals made by Everton in the previous summer.