“The most memorable protagonists in plays are significantly flawed in some way.” Do you agree with this statement? Respond with close reference to Death of a Salesman.

There is a lot of potential directions that you can go with this writing prompt. You are certainly able to disagree with the statement and say that the most memorable characters are those that are perfect. I don't recommend doing this. First, I legitimately think that disagreeing with the prompt is a more difficult task. Second, agreeing with the prompt and supporting the prompt is likely what your teacher is looking for.
In Death of a Salesman, Willy Loman is at least two things. He is flawed and also quite memorable, so he works as a good supporting character to your writing prompt. A direction to go might be to go the standard hubris angle; however, I do hesitate to fully claim that Loman suffers from excessive pride. He's definitely delusional. In his mind, he's hugely successful, but he also suffers from serious self-doubt. All of this combines to form a character that comes across as extremely arrogant at times, and that makes him very memorable.
If you need to support the prompt with other characters, I might look to another Miller play. You could pick John Proctor from The Crucible. I would explore his flaw of lusting after Abigail or his pride in his good name. He ultimately lets himself be killed to protect his name. Macbeth would be another great character to explore as his flaw would the flaw of unchecked ambition. Ambition is good, but he lets the means justify the end, and he's been a memorable character for 400 years.