Who is the antagonist and protagonist in Harry Potter and the Sorcerer's Stone and why?
The protagonist of story or novel is the hero -- the person we are rooting for or that is the focal point of the story. Sometimes they can be horrible people, but in this case, the protagonist is Harry Potter. He is the very special boy who is finally learning the truth about his talents and his place in the world -- a wizarding world he knew nothing of until now.
The antagonsist(s) of a story or novel are the characters who create the conflicts for the protagonists. There are several characters who fit this bill in the novel.
The most obvious, and prevasive, is Voldemart who is determined to take over the magical world with his evil ways. He must destroy Harry in order to do it, and while Harry likes Hogwarts school and his new friends, he doesn't know whom to trust.
Other antagonists in this novel: The Dursley's -- Harry's aunt and uncle who are his guardians, but who treat him very poorly; Draco Malfoy -- the bully from Slytherin; Professor Snape whom Harry and the other kids think is trying to hurt Harry (though in the end it is revealed that he was protecting Harry); Professor Quirrel, it is revealed, is the true "bad guy" of the novel as he is the working working for Voldemort and who wanted the Sorcerer's Stone.
The main protagonists of this story is obviously the Boy Who Lived, Harry Potter. That being said Ron Weasley and Hermione Granger are Harry's close friends and each have integral roles throughout the series. Harry Potter is only as good as his friend group and he definitely accomplishes more with them then without.
As for antagonists- there are several. The chief villain is obviously Voldemort. He Who Must Not Be Names is not only responsible for the deaths of Harry's parents and countless others before his birth, but continues to his bid for power while experiencing various levels of mortality.
As a result of Voldemort's shifting living status, he relies heavily on the use of various wizards to help sustain his viability. In the case of the Sorcerer's Stone, this falls to Professor Quirrell who is acting as the Dark Lord's host. While Quirrell is seen as a sniveling almost sympathetic character, he is still acting out the will of Voldemort as he assists in sabotaging Harry's schooling.
Finally, the Dursley's are most definitely another group of protagonists. Aunt Petunia acts out her jealousy of Lily be treating Harry miserably. She pamper's her son and emotionally and physically abuses Harry. Her fear of Harry turning into his parents cause Uncle Vernon and Aunt Petunia to hide Harry's heritage from him. They also try to prevent him from going to Hogwarts. They try to manipulate Harry throughout the beginning of the story.
The protagonist of a story is the story's main character, which, just as the title suggests, is Harry Potter. The antagonist is the person who works against the protagonist in the main conflict of the story. Although the antagonist does not necessarily have to be "the bad guy", in the case of Harry Potter this standard "bad guy" image continues. In the series as a whole, the antagonist is Lord Voldemort as Harry continuously comes into touch with Voldemort while trying to save the Wizarding World. However, for the first book alone, the antagonist could just as easily be Quirrell. As he shares a body with Voldemort, it is difficult to separate the two characters when discussing who the antagonist is. The qualities of the antagonist were the intentions of Voldemort carried through by the actions of Quirrell.
In a broad sense, the protagonist is Harry and the antagonist is Voldemort. However, Harry doesn't really come into contact with Voldemort until the very end. He doesn't even realize it is Voldemort until the very end. So its tempting to say that Quirrel is the antagonist. Yet, "no one suspects poor stuttering Professor Quirrel". Harry suspects Snape throughout the whole novel but it wasn't Snape!