Same here, i don't hate Ran, but i extremely hate ShinRan.
As for character that gets on my nerves, none. But the closest one should be Kogouro, his stupid deduction (especially in movies) really annoy me. I find Sonoko and Genta are ok. Sonoko could be annoying sometime but she's really nice and open-hearted for a rich girl. Genta is annoying, but he's a kid so that it can't be helped, i think he's still much better than a real 1st grade kid.