What do you think is the most important theme of Death of a Salesman and why? Discuss the meaning/significance/function of the scene of your choice in relation to the theme of the play.

I would say that Death of a Salesman's most important theme is to be true to yourself and work to develop what you are good at and drawn to, rather than trying to get rich quickly. Willy Loman buys into a version of the American Dream in which he believes he can sit in a hotel room in his slippers, work the phones as a salesman, and earn money that will come rolling in effortlessly. This is an illusion. Tragically for him, he fails as a salesman, ending life with very little money beyond his life insurance policy, passes on his failure to his sons, and misses out on what he might have really enjoyed doing. This is the most important theme because your life can be wasted and the people you care about most hurt if you put earning money ahead of finding and working at a meaningful vocation. Willy has become divorced from reality in his pursuit of an illusion he can't give up.
To illustrate this theme, I would pick the scene in Act II where Willy is having a conversation with his boss, Howard, about getting a raise and an easier work schedule. As Willy persists in thinking he is valuable as a salesman, Howard is trying to fire him. Willy, however, falls back into fantasy, telling Howard the following story:

Oh, yeah, my father lived many years in Alaska. He was an adventurous man. We’ve got quite a little streak of self reliance in our family. I thought I’d go out with my older brother and try to locate him, and maybe settle in the North with the old man. And I was almost decided to go, when I met a salesman in the Parker House . . . . And old Dave, he’d go up to his room, y’understand, put on his green velvet slippers—I’ll never forget—and pick up his phone and call the buyers, and without ever leaving his room, at the age of eighty-four, he made his living. And when I saw that, I realized that selling was the greatest career a man could want.

This passage shows that Willy could have done something else, make a life in Alaska, and perhaps pursue what we suspect he really loves: gardening and working with his hands. Instead, he falls for the illusion represented by Dave Silverman. Willy's problem is that he has never been like Dave Silverman (if Dave Silverman's story is real) but can't accept that, so strong is the power of his dream.