Well, I have good news for you! You don't actually need to "decide" at all, as if you read the text, you will find the answer clearly in Act V scene i. This of course, is the final scene of the play and the denouement where all of the conflict and the problems that have heightened up until this point are straightened out, happiness is restored and confusion is dispersed. However, to analyse this play effectively you need to ask yourself which characters are excluded from this circle of happiness at the end. One of the characters who is definitely left out in the cold as it were is Malvolio. He, out of all the characters in the play has been "most notoriously abused" in the words of Olivia by the excesses of Sir Toby Blech's revenge, and as he is brought into the hall before everyone out of his prison, he declares his intent to gain vengeance:
I'll be revenged on the whole pack of you!
This presents a jarring note that does not fit well with the otherwise happy ending that the play presents us with. Thus the answer to your question is Malvolio.