Looking on the internet deeply has found these results:

Dear Sir is the most popular phrase on the web.

Respected sir 1504,000 results on the web	Dear Sir 124,700,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
They respected him, right?Yes, sir. I could tell that he respected you, sir.	Dear Sir or Madam: Soon your son will drop from the sky to engage and defeat the enemy. The process to get to make the sculpture was interesting, because I wrote to Balsara, who produces that air freshener called Odonil, saying, Dear Sir, I am an artist. This is my catalogue. Well then... Moucheboume Dear Sir, Space... OK, write: "Dear Sir", comma, new line, Dear Sir, it frequently happens we are unknown and strangers to ourselves and our identity travels, so to speak, incognito. Dear Sir, I don't care. Dear Sir, only you and three other people were able to resolve the puzzle. Dear Sir, Having learned of your courageous endeavor to visit the land of the Bolsheviks, we are sending you some New York magazines that depict the barbarous state of Russia today.

Respected sir

1504,000 results on the web

Dear Sir

124,700,000 results on the web

More popular!

Dear Sir or Madam: Soon your son will drop from the sky to engage and defeat the enemy.
The process to get to make the sculpture was interesting, because I wrote to Balsara, who produces that air freshener called Odonil, saying, Dear Sir, I am an artist. This is my catalogue.
Well then... Moucheboume Dear Sir, Space...
OK, write: "Dear Sir", comma, new line,
Dear Sir, it frequently happens we are unknown and strangers to ourselves and our identity travels, so to speak, incognito.
Dear Sir, I don't care.
Dear Sir, only you and three other people were able to resolve the puzzle.
Dear Sir, Having learned of your courageous endeavor to visit the land of the Bolsheviks, we are sending you some New York magazines that depict the barbarous state of Russia today.