Allardyce for me.
It was his whole attitude plus approach to football, he more or less admitted defeat against the top 6 before kick-off and that run of 0 shots on target was absolutely embarrassing.
Even when we were comfortable away from relegation, he still treated us like a relegation threatened club and showed no signs of wanting to take us to the next level - however this could be because deep down everyone including him knew that he was only going to be there for 6 months because Moshiri caved in to his contract demands. He had an opportunity to get rid of the "Big Sam survival specialist" tag but he was worse than I could have imagined.
Koeman comes second, apart from using Twitter he made little effort to engage with the fans and embrace the club. Even though you could argue he was hampered by having Walsh as director of football, for an experienced manager I am surprised at just how poor our transfer strategy was and this obsession with signing "number 10s".