There's been a few contenders. There's worst players - as in terms of ability - and there's worst signing - which I think needs to take into context fee as well as performance.
In terms of signings I think Sandro, Bolasie and Nyarko would be hard to beat. With those players their Everton career is known and the monetary cost to what they gave isn't going to change.
Someone like Iwobi could end up on that list. Though he has ability and has time to turn things around.
Worst player in terms of ability is probably Brett Angell. Tony Thomas and a few of the GKs are up there as well.
Someone like Rondon isn't showing anything at the moment but he has some sort of pedigree. Tony Thomas and Brett Angell were never at the level to play regular PL football