This is a play which does not have any obvious central character. Rather, Hansberry chooses to focus equally on the four main characters, Beneatha, Walter, Ruth and Mama, exploring their different views and perspectives to what is happening in their lives. This means there is no clear cut answer to the question above. However, it is possible to argue that Walter is the main character. This is because it is he who undergoes most development and change in his characteristics during the play. Remember how he starts off the play immensely frustrated because of Mama's refusal to give him any of the money to start up a liquor store. However, after he has been given the money and has seen it taken away he is still able to exert himself and show his strength of character to Mr. Lindner in the following quote:
[We] have decided to move into our house because my father—my father—he earned it for us brick by brick. We don’t want to make no trouble for nobody or fight no causes, and we will try to be good neighbors. And that’s all we got to say about that. We don’t want your money.
He therefore could be argued to be the main character in the way that he shows growth, maturity and development and is able to not be dominated by his own failures. Hansberry allows him to speak these words with tremendous dignity as he remembers his father and how hard he worked for the entire family.
Each of the adult characters in A Raisin in the Sun is burdened with a significant decision. These decisions constitute the drama of the play, in large part, and for this reason we cannot say that there is a single main character in the play.
Walter and Mama are the most likely characters to be considered as the main figures of the play because the decisions that they make can be argued to be the most central to the main conflict of the the play. Walter and Mama are the ones who decide if the family will stay or move into a new house.
Outside of deciding the family's geographical future, Ruth has the power to decide whether or not to keep her baby and Beneatha is engaged in a subtle and importance process in deciding how she will identity herself going forward. Her identity is crucial to the family as well, because in choosing who to be she is also choosing to keep or reject both the love and the values with which her mother has raised her.
This is a play about a family and it's struggles. No family member narrates or receives more than a balanced share of stage-time (or page-time). For this reason, making the argument that there is no single main character is easier and more accurate than making an argument that either Mama or Walter should be viewed as the central figure of the play.
However, again, Mama and Walter are perhaps most significant in their roles in the family with Walter being, formally, the head of the household and with Mama standing as "the source of the family's strength as well as its soul".