A bad transfer for me has to include poor decision to get in the first place. Hazard is more bad luck than he didn't fit the system or he cost more than he was worth at the time etc.

Lukaku's gonna end up up there I reckon.
But at the moment it's got to be Coutinho. It was a panic buy after Neymar left and they wanted to save face by buying the biggest money player they could, he didn't fit the system, he was played out of position, given inconsistent starts, started performing badly and just never got back to his old ability whilst being on ridiculous wages and costing ridiculous money to begin with. And years on are still struggling to recoup any money from it at all back.