The Incumbent Doctor (aka. The Current Doctor) is whoever the Doctor is right now.
It's one of those unwritten (and largely ignored) rules in fandom that you have to be (at least somewhat) loyal to the Incumbent Doctor even if you don't like him, otherwise the Beeb might pull the plug again like they did in the 80's when fan opinion soured on our beloved Pie Lord. Fortunately on 4chan no such restrictions exist and you're free to hate any Doctor you want, although you will be bullied if your opinion is shit, which we all know it is.
Alternately you can just hate the Doctor before he's even started.