It's hard for me to pick the worst habit of his, because they all come with an upside of showing his true inner self, which deflates the whole charade of his. For example, when he legit thinks he's funny even though he isn't, it comes with the consequence of cementing how dumb/immature he is, and his rep reflects that accordingly.
Basically, it's hard to be irritated by anything in particular when he doesn't truly get away with any of it, since he's not clever enough to pull that off. There's always something about it you can laugh at him over, even the fact that he threatened a little girl, because he was too much of a pussy to do it in front of her. Or when he lied about Jasper being sick for pitybux, because he got scared and retconned it immediately.
Phil would be way more of a rage-inducing cunt if he stuck to his guns from actually having a spine and/or was a little bit more intelligent.