Trip is just boring. I would give him the most boring character title, or he would at least be in the top five. Though there were times he put down Ash saying that he lives in the boonies and saying stuff like he wasn't really a threat, but that's his opinion really.
Gary only really seemed like a jerk to Ash. He picked on him here and there and was a prick. But now we've seen that he's got some character development, so he's not so bad now from what we've seen from him in Johto & Sinnoh.
Koji just really wanted that Cindaquil. Even though he's kinda a sore loser, I don't think we really know him to say he's the biggest jerk.
Paul just cared about being stronger, & Ash & co. just bothered him for his training style. While he was cruel to Chimchar, it was what he wanted (till he dumped it on Ash cause it lost). Even when he released his pokemon for losing, at least they are free to do what they want now. He might've changed once Ash beat him, but we haven't seen him to know.
Harley harassed a girl because of a cookie he doesn't even know stole it, and can't ever get over that fact no matter how nice she is to him. He's also very self-centered.
Damian like everyone else said, didn't even give a **** that he almost killed his Charmander, then once he saw that Charmander was worth while, he tried to take it back from Ash.
Conclusion for biggest jerk (for poll): Damian > Harley > Paul > Koji > Gary = Trip
Burgundy was another I was thinking of. Don't think she beats Damian, but seriously 90% of the time she's shown she's YELLING cause she's always pissed off at Cilan, even when he didn't do or say anything mean to her. She also tried to get Ash to get rid of his pokemon, because she personally didn't like them (she wasn't really being that nice to them, saying mean things about them and all, so to no surprise they attack her), or it could've because of Cilan really (since she saw Ash with him). She also forced Ash into a room to do her evaluation.