Luffy: One look at the birthday Oda gave him (Children's Day) and you quickly realize that Luffy is intended as the innocent man-child fulfilling a destiny too rigorous for a child and too naive for an adult. Please forgive my lack of umlauts over naive.
Zoro: Mr. Bushido, the one who is so manly that he hasn't cried since I don't know..when Mihawk handed him his ass? Zoro has his own code of honor, and he sticks by it. He is ethical, honorable, and driven to succeed. Maybe a little too driven. He can be cold, but he still cares about his crewmates and their safety. Macho, certainly. A "true" man? depends on your cultural outlook.
Sanji: Zoro's foil, Sanji has no mercy on any man that insults him but will let himself be abased beyond all indignity by a woman. This is not to say that he stands there and lets every villainess thrash him without lifting a toe; He did block a few of Califa's attacks
! and disarmed Lola/Laura without hurting her, or allowing Nami to get hurt. He also stands up for Usopp when he's in trouble–maybe he still feels guilty for clocking him by mistake back at Cocoyashi. His code-to feed the hungry, no matter what their disposition toward him or vice versa- he has upheld well for nearly four hundred chapters. His Mellorine Mellifluousness is his outstanding character flaw, but all true men have at least one flaw--don't they?
Usopp: His dream of being a 'Brave Warrior of the Sea' is lived out every time he doesn't abandon his friends and run screaming back to Syrup Village every time the opportunity presents itself. However, to be honest, the opportunity hasn't presented itself often. The fights he wins are those that he cannot run from with impunity, or those that he would feel shamed by slinking away from. He also suffers from an almost manic-depressive swing in behavior; as confident as he shouldn't be one moment, and agonizing over his own cowardice the next. Usopp is a flawed character, but his flaws are human and we the readers can identify with him somewhat. But it is his humanity we identify with, and not manly behavior. If Luffy is the man-child, Usopp is the perpetual teenager; unsure, cocky, angst-ridden and light-hearted all within the same hour.
Franky: In Japanese, the ideographic concept behind otoko (man) is "able to feed ten mouths". Given the number and size of the Franky Family, I'd say he pulled off that feat rather well. Brash, obnoxious, crude, and melodramatic, Franky seems incredibly immature when compared to his contemporary, Iceburg. Even compared to Zoro or Sanji, Franky seems immature. Yet there are times when he shows kindred feeling without mawkishness or silly poses. These are the times when he is at his manliest. Interestingly enough, these are also usually the times when he is with Robin. Steve Reesel will have a field day with that. A true man? no. A man's man? no. A ladies' man? no. Franky is his own man-and that's about all a man can hope to be.
In conclusion, there are no good "true" men aboard the Thousand Sunny–there are only men good and true. I would hope that is enough.