Who are the main villains in Much Ado About Nothing?

Well, the main "baddie" or villain of the play is
definitely Don John, the bastard brother of Don Pedro, who at the beginning of the play
has just had his rebellion against his brother's rule smashed and has been welcomed back
into his brother's court. However, it is clear from Act I scene 3, when we first see Don
John out of the public gaze with his two henchmen, Conrad and Borachio, that he is
anything but resolved to his defeat. Note what he
says:

In this,
though I cannot be said to be a flattering honest man, it must not be denied but I am a
plain-delaing villain.

It is
clear that he is incredibly displeased by his defeat and his own low social standing
compared to that of his brother's, and thus hatches a plan to attack one of his
brother's most trusted friends and slander his bride. Note what he says about the idea
of attacking Claudio is such a subtle way:

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This may prove food to my displeasure. That young
start-up hath all the glory of my overthrow. If I can cross him in any way, I bless
myself every way.

Thus the
stage is set for the deception of Claudio and Don Pedro and the slander of Hero,
engineered by Don John, the definite villain of the piece, ably assisted by Borachio and
Conrad.