Why do people like Tony Stark more when Captain America and Natasha Romanoff are ready to sacrifice themselves anytime, while Stark hesitated and was scared to sacrifice himself?

Because Tony actually did sacrifice himself despite his fear.

Captain America and Black Widow were totally ready to sacrifice themselves. They had everything to gain and nothing to lose. They had no family, no friends except for each other (considering Falcon and Bucky were dead for a while now), no love interests (Black Widow couldn't have kids either way, and Cap's true love was all the way in the past), and were wanted criminals in the U.S.

Tony Stark was a dad. And a husband. He had a loving family and a home, he was vastly successful with tons of money and his vast intellect. He had a daughter he wanted to be there for, and a wife he wanted to protect. He had family that he cared for more than ANYTHING. Hell, even at the beginning he was terrified of meddling in all of it because he was scared that his family would die or that he would die and leave his family alone.

But he did it anyway. He found out how to fucking time travel. He invented a new fucking infinity gauntlet. He harnessed ancient artifacts from beyond time and quantum physics. He risked everything to save the universe. And in the end it cost him his life.

Tony Stark was scared because he had everything to lose. But he stowed his fear and went in. And, because of him and his sacrifice, the war was won. Not only did he sacrifice his life, he sacrificed a happy life for his family, and his wife and daughter.

That's why Tony Stark is the mightiest Avenger.