1. Luffy - He's oblivious, but he gives everyone hope to persevere
2. Franky - He's a freaky showman surfer inventor, who can hammer together giant stairs in the middle of combat, and has weapons powered by Coca-Cola!
3. Nami - She's sneaky and thinks on her feet. Too inconsiderate towards her friends when she wants to make money though. Nabiki Tendo definitely had an influence here, much like Ryoga Hibiki had with Zoro.
Honourable mentions:
Zoro: I like that he always works hard and is determined, but he's too willing to start killing people for my taste (I'm thinking of those 50 bounty hunters at Whiskey Peak that he had pretty outclassed, but that may have been a mistranslation)
Chopper: Really idealistic and applies his brain, so he's the type of character I usually like, but his abilities are usually pretty lame, and he doesn't get any cool moments. Maybe if he started using that giant form more?
Garp: He's a really tough, laid-back, and funny old man, but I don't quite get him.
Mister 2: He's funny in a weird way.
Smoker: He's tough, righteous, and honourable. The kind of law officer who protects people.