Every member of the Strawhat crew was held in the highest possible standards by their captain Luffy.
He didn't care any about their back stories: who exactly they were, where they came from… If he thought they were good people, they were good enough to join his crew.
And when he wanted someone to join, he WOULD have them. Many of the Strawhats found that out the hard way. He was a very odd guy in that sense.
Luffy also never thought once about picking favorites.
He had a First Mate, a Navigator, a Sniper, a Cook, a Doctor, an Archeologist, a Mechanic, and a Musician who he thought made up the best crew EVER. And if anyone ever hurt any of them, they would have to face him, and he would have no mercy for them.
He never thought about picking favorites, but there was someone on the crew who seemed to get a different kind of attention than the others…
Luffy protected his dream with his own life.
Luffy wouldn't stand for anyone, ANYONE getting in the way of that man's goals.
Luffy counted on him to be standing right beside him when he became Pirate King.
Luffy knew he would follow him anywhere, no matter what.
Luffy knew if anyone ever said he didn't want to be apart of his crew anymore, he would say:
"Bullshit."
Luffy cared so much for him that it seemed he didn't care at all.
His First Mate, Zoro.
The crew had no doubt noticed these feelings when something happened to Zoro on the battlefield. Luffy had never been so angry and scared in his whole life since Mihawk almost killed Zoro.
Another time, every single one of his Nakama was being taken away from him, and no matter how much he struggled he couldn't do anything. The last one to be taken away was Zoro.
Luffy couldn't stand it. He went past his limits trying to save Zoro. And he knew that the swordsman would do the same for him.
That's just how it was between them.
They only ever worried about each other on sparse occasions. And they knew when the other was having troubles. Without even saying a word, they could devise a battle strategy and go through with it without missing a beat.
There was a connection between Zoro and Luffy that nobody else had in the crew, and neither of them knew exactly what it was. All they knew was that they would follow each other to the ends of the universe.
Luffy didn't pick favorites, and yet… Zoro was his favorite.
Because after all, the Pirate King couldn't go on without the Greatest Swordsman In The World!