Still need to know?
The main characters are Monkey D. Luffy, Roronoa Zoro, Usopp, Nami, Sanji, Chopper, Nico Robin, Franky and Brooke.
As for the most popular ones, that's a toughy since there are so many characters in the universe (I can't name all the characters I like in the universe)... but if I would have to give you a reference, go to
[link] and look that the 4th poll in ascending order. Sure the poll is from 2008, but a lot of those characters are still active in the show even today.
If you need anymore info, I'll try to get back on DA at least once a day, but no guarantee. College hates me right now. >_>;