You're really asking for it. One mistake and people can't wait to jump on him. In what world is Oblak and especially Alisson better than De Gea? Where was Alisson before last season? Only Neuer can rival him at the moment, and it's not as if he hasn't had his fair share of feckups.
Not one mistake.
I think they are all around the same level but I would rank ter stegen as the #1 GK from last season.
DDG Oblak Alisson perfromed at the same level last season.