Nami's voice really didn't bother me, but her kid voice was so annoying.
Zoro, or "Zolo" sounds too old to be 19.
Luffy bothers me a lot more than them, though. Especially his laugh.
For the most annoying voices, Usopp and Sanji are the winners.
Usopp: Sounds like Sonic the Hedgehog if he was castrated.
Sanji: Sounds like a stereotypical New Yorker, or Dr. Girlfriend from The Venture Brothers.