Donny's transfer has been an abject failure, but it didn't really change much for the club, positive or negative. The fee, while not small, wasn't particularly large and his salary isn't that high. And he wasn't really brought to be an integral part of the team, so his absence wasn't really felt. After he is gone this summer, he will be forgotten. It was a very strange transfer, definitely one of the worst transfers, but nowhere near the worst.
Sanchez's transfer actually damaged the club long term without any tangible return. His salary prompted other players to demand more and that's how we ended up with a bunch of unsaleable players now. By far the worst transfer in the history of the club. It was a travesty.
I'd also put Sancho and presently Antony above Donny. They both cost unspeakable amounts, came in to be integral players, and both have completely failed to deliver, leaving the RW position in a dire situation once again. Antony's transfer has put a hole in our budget this summer, and Sancho's criminal salary makes him unmovable.