In the Loman family, Biff is the person who develops into a mode of self-critical analysis. He may not be truly a "critical thinker" but he comes to apply a certain skepticism to his attitudes and presumptions.
In challenging his former worldview, Biff becomes more of a realist than Willy and Happy.
This is an intriguing question in light of this play. Critical thinking requires analysis, yet nearly all the main characters base their critical thinking upon myths. Willy can employ critical thinking skills; he commits suicide to provide Linda, his wife, with money. Rationally, he knows that is the only way he will make that much money. Ironically, it is the wrong thinking as his assumption is that money is more important than life! The myth of heading West and growing things to become part of the land is the basis for the thinking of both Willy and Biff. Again, ironically, Willy's land is an unproductive garden and Biff will have a difficult time whether he goes West or not. Perhaps Linda is the only character who really never thinks about much until the end of the play when she realizes in Requiem that the house is paid for, but she has lost her husband. Remember, we can all think critically;however, the larger issue is just what perceptions and assumptions we base our thinking upon! There are some great articles at enotes Salem site!
I think this would be a great question for the discussion board.
Here's www.dictionary.com 's definition of "critical thinking":
the mental process of actively and skillfully conceptualizing, applying, analyzing, synthesizing, and evaluating information to reach an answer or conclusion.
I don't see any of the characters as ones who are "critical thinkers." Each of the main characters, Linda, Biff, Willy, and Happy, live in their own little "worlds" that aren't reality. To think critically means that a person must be able to effectively analyze, and I don't think any of the four characters are capable of doing that without it being completely altered by the false realities they are living.
In the end, the critical thinker is Charley because he sums up Willy in the requiem. Happy has the least ability to think critically; he's a follower.