A toss-up between Timmy turner and Angelica pickles.
timmy-In the last season,when butch(supposedly) left the show for Danny phantom,he became more of a whiney little bratty snot,cosmo became a jerk,and wanda became miss nags-a-lot. I actually rooted for vicky in the last season.(dude,she's freaking hot!)On top of it all,he never gets the bitter end of the deal,and he has the whiniest voice that makes pamela anderson seem like AC/Dc's greatest hits.
And must I say anything about Angelica,ladies and gentlemen? I think not.