Captain America bc he is a friendly, loving, leader that has a shield and he helps other people and i kinda have a huge crush on him... <3
Hes the protector and well he thinks about the people not about himself (sorry ironman).
Every Avenger has their own personal flaws and no one is perfect, but Captain America always does the right thing and stands up for his beliefs. I see him as the Superman of the Avengers, even though he isn't the most powerful, he definitely deserves to be the leader.