Yes, by a mile. There are other transfers, such as Di Maria, Darmian, Lukaku, etc but on those we could get our money back, or at least most of it.
Alexis wasnt just a disaster because of the money paid, but it disrupted the squad. Martial being relegated to the bench, and our wage structure totally messed up, being honest we are really lucky he had a good spell at Inter and we could move him there.
I think is if not the worst transfer in the premier league at least is one of the worst 3. Along with Shevchenko to Chelsea and Andy Carroll to Liverpool.