Which character is most to blame for all of the witchcraft hysteria that breaks out in Salem in The Crucible?

Abigail Williams is the most to blame for the witchcraft hysteria that takes hold of Salem.  She participates in witchcraft rituals in the woods, theoretically causing her cousin (and Ruth Putnam) to become ill from guilt and fear and allowing her uncle to find her and the other girls dancing and conjuring.  Her actions begin the entire problem.  Betty Parris wakes for a few moments in Act One to scream, "You drank blood, Abby!  You didn't tell [Reverend Parris] that! [....] You drank a charm to kill John Proctor's wife!"  Clearly, Betty is stressed out about their activities in the forest, and she is anxious about her father finding out.  If Abigail hadn't actually been engaging in these illegal and immoral activities, the hysteria would never have even begun.

Further, Abigail makes the first accusation when she names Tituba as a witch.  Once Abigail becomes the subject of Mr. Hale's questions, she panics.  In order to redirect suspicion away from herself, she cries out, "She made me do it!  She made Betty do it!" and she blames Tituba for "mak[ing] [her] drink blood," laugh during prayer, and sleepwalk naked.  Putnam and Parris then clamor to hang or beat Tituba to force her to confess, and she does what Abigail did before her: accuse someone else, someone who the town will believe could be a witch.  Thus, Abigail starts the chain of accusations that ignited the hysteria.

Finally, Abigail turns Tituba's attempt to save her own life into an opportunity to accuse others in the town and create her position of authority in the trials.  By the end of Act One, Hale is blessing Tituba and calling her "God's instrument," one who has been specially selected by God to "help [them] cleanse [their] village."  Tituba has become the center of attention, has acquired a position of authority (at least, relative to her position before), and Abigail wants this too.  She "rises, staring as though inspired" and she yells out, "I want to open myself!" and she repeats the names of the women Tituba named, adding one more.  Indeed, she has been inspired; she seems to realize that she will be believed if she falsely accuses others, and we might imagine that she now sees her opportunity to accuse Elizabeth Proctor.  She proceeds to falsely accuse three more women before the end of the act.