Hard to say, really.
Martinez first season was the most I've enjoyed watching Everton since I was a kid in the '80's, but there's probably an argument to be made that he started the current rot.
Koeman was an absolute catastrophe. Not really sure what happened but there seemed to be no strategy at all, the fees thrown at average players were insane. £28m on 'kin Bolasie FFS. The squad ended up entirely unbalanced and in need of a complete overhaul.
Large Samuel did, on paper, a reasonable job, but the man is a stain on football and should never beem invited to tarnish our club, and the football was bad enough to make yoyr eyes bleed. I refused to go to Goodison while he was the manager.
I never wanted Silva, I don't really understand why Moshiri pursued him so desperately, there was nothing on his CV to say he was the man for the job, and he's also failed to get a tune from these players and spent big bucks on dross.
I've gone for Martinez just because of that first season.
Ultimately though, I don't think any of these managers are entirely to blame. Moshiri and Kenwright don't seem to have the first clue what they're doing. There also seems to be something cancerous eating away inside the club. I don't know whether it's the amount of "old boys" we employ, I've heard it said that some of them are utterly toxic individuals, but the current malaise is down to more than just poor mangerial appointments.