Ambition
Each member of the crew has it's own ambition; little is known about the ambition of those outside.Luffy - to become the Kaizokuo - the king of pirates, which as per his definition is the person with the most freedom on the seas.
Zoro - to become the best swordsman in the world.
Sanji - to discover All Blue and become the best cook.
Nico Robin - to uncover the secrets of the Void Century and, become the best historian and scholar.
Franky - to become the best shipwright and carpenter of the Kaizokuo.
Nami - to draw a map of the world.
Chopper - to become the best doctor.
Usopp - in his own words, he wants to be a brave warrior of the sea, but according to his talents, he is poised to become a world class sniper like his father.
Brook - he seeks to conquer Grand Line but after his rebirth, his ambition is less clearly defined. To meet with Laboon again, perhaps?
Based on ambition and the effort taken towards it, Zoro is the one whose combination of seriousness and how close he is to his target is closest. In fact, among everyone's definition of ambition, Zoro's is the one which entails violence by default. One does not simply play with swords and become the best swordsman in the world. One could argue how Usopp's doesn't qualifies, but if you threw a gun in Usopp's hands and asked him to shoot someone, I'd be hard pressed to be convinced that he would.
Portrayal of character
Being the captain of the ship, Luffy steps up at the most important points in the story; most notably being at Eneis Lobby when he asks Sogeking to shoot the world government's flag down. What. A. Scene.This scene is testament to the seriousness that Luffy has over his ambition of becoming the Kaizokuo. It is only matched by Zoro's rebuttal when Luffy says he is going to fetch Usopp who leaves the crew, fighting over the Merry. Another great scene. I'm yet to see another character from the Mugiwara no Ichimi displaying the same amount of seriousness.
Aiming to be the best swordsman in the world, Zoro's character comes with an implicit smugness and blood thirstiness. The latter makes his, a very compelling character amongst the Ichimi(most of whose fights involve minimal or no blood thirst). There are many fights where Luffy's life is on the line and he exclaims that he is going to kick the antagonist's ass, but he isn't out for blood. Zoro is. Notable scenes are
Ashura at Water 7 - "You got cocky and said too much, you bastard!"
and Daishinkan at Punk Hazard - "Have you seen a fierce animal but were convinced that it wouldn't attack? I haven't!"
Near death scenarios
With respect to near death scenarios, Zoro has had the most that prompt you to say, "Man, that guy got really fucked up by this dude." The fights with Mihawk near the Baratie, the fight with Daz Bones and the portion where he decides to absorb Luffy's damage from fighting Odz against Kuma are some of the best sections of the story without contention. In each of these, you get to see Zoro being visibly on the edge of death. I'd be hard picked to choose between the three as Zoro's best.Zoro's biggest badass moment
Fights
Chimping off the previous section, Zoro has had some pretty amazing fights and opponents. Daz Bones and Kaku are among my favorite fights in One Piece. I've always felt that the amount of detail in fights was less (much less!) compared to Naruto which has some of the best fight choreographies till date. But these 2 fights compensate me for it partially.Zoro's inner peace moment where he can cut steel and not cut a leaf leaves(excuse the pun) me with goosebumps how many ever times I watch it. The huamndrills' challenge comes close.
Inner peace
It is difficult to have a character with such a dark and intimidating personality yet make gags, which the grand master Oda-san pulls off amazingly well with Zoro's lack of sense of direction. Zoro is an amazing character and it's tough to not have him in your favorite characters list. I'm sure you'll agree. :)
