Online:
Kasumi, Christie & Phase-4 are the ones I seriously hate fighting online. You can get away with them so much online.
The only spammers I don't like dealing with in a fight with a specific character:
Hayabusa only using teleports and space the whole time ; Ayane players that use her 1P+K every single second ; Hitomis 6T ; Gen Fu & Eliot players only using 236T as a grab
Offline:
Brad Wong, Mila, Zack & Pai mirrors
What exactly is wrong with that? That's what you generally base those character's gameplay around. Learn to defeat them. Brad Wong's actually not that bad to deal with if you know what he can do.
For me currently:
Offline: No one really
Online: Everyone