Captain, Monkey D. Luffy. Swordsman, Roronoa Zoro. Cook, Sanji. These three are the strongest fighters of the Straw Hat crew, so much so, that they're known as "the monster trio". Usually, these 3 end up fighting the strongest enemies. Even some people have wondered, in the case of Zoro, how he can be so strong and yet not be the captain.
Zoro, is basically a master swordsman. He's not yet the strongest, and he still has much to learn, but in the One Piece world not many people can defeat this practitioner of the 3-swords style.
Sanji, only uses his legs, since his hands, as he claims, are only used for cooking. But don't let that fool you, since it's not at all a handicap. Sanji's legs are incredibly strong, enabling him to not only be an incredibly fighter with his kicks, but to also run in water, among other things.
And Monkey D. Luffy is simply a genius fighter. Able to come up with new moves on the spur of the moment. He goes with the flow of the battle, using his imagination in many different and ingenious ways, making him a formidable opponent. Not only that, but his entire body is a weapon, strong and fast and thanks to the powers of his devil fruit, his natural strong body reaches even new levels of strength.
So one last word of advice, if you're a female swordsman, eating a whole lot of meat, try not to meet with them. It won't be pretty if you do hahaha
One Piece and everything related to it are owned by Eiichiro Oda, Shueisha and Toei.