As these forums can often be informative, I'd like to make my contribution of something I haven't spotted anywhere.
To taint your imagination of the voices whether in the manga or anime, here are the actual people behind the scenes doing all the voicework in the anime:
Luffy - Mayumi Tanaka
Zoro - Kazuya Nakai
Nami - Akemi Okamura
Usopp - Kappei Yamaguchi
Sanji - Hiroaki Hirata
Tony Tony Chopper - Ikue Ohtani
Nico Robin - Yuriko Yamaguchi
Franky - Kasuki Yao
And there you are, the voice-actors of the straw hats. Images courtesy of animenewsnetwork.com.au/