What an odd and interesting question. Well-conceived 
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Thinking on it, I feel my favorite is a nice and thorough verbal lashing. It might be called a Breaking Speech, a "The Reason You Suck Speech," or a Hannibal Lecture. The details differ, but in brief, it's when one character uses words to tear out another character's soul and show it to 'em in all its wretchedness.
One thing I like about it is that it's easy to do badly, but also easy to do very well. It's a powerful form of internal character study when it works, and more often than not, the person on the receiving end definitely has some major flaws they aren't addressing. It also puts the receiving character at an unmistakable crossroads: either learn and grow from the truth in the speech, or shun any potential revelations. For the writer, it's even an opportunity to flip a character from heroic to villainous or vice versa.
Finally, as a wordy bastard myself, I do love it when characters wax philosophical in an effective manner, though some of these verbal lashings can be extremely brief.
