: The basics of making a "reason you suck speech" that doesn't feel contrived "The reason you suck speech" is a trope, that is described as: A character gives a speech on why someone or something

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In short:
Tell the truth.
What do I mean by that? Here's some writing advice from one of my favorite British authors:

Even in literature and art, no man who bothers about originality will ever be original: whereas if you simply try to tell the truth (without caring twopence how often it has been told before) you will, nine times out of ten, become original without ever having noticed it.
- C.S. Lewis

Truth, in this context, is what is true to your characters.
Try to focus less on making the speech sound "actually good", and more on how the speech is going to make the receiving character feel. How are you going to achieve the main character feeling like they suck? If you're going for a serious (not comic) dramatic effect, your ammunition should draw heavily on the flaws of the "you suck speech" recipient.
Take your GLaDOS quote, for example. The reason this quote is a joy to read is because 1) it's true to the characters (GLaDOS's existence / the testing facility is built on making up facts to influence the test subjects) and 2) it contains a stab of truth: the test subject is, in all likelihood, a clone who doesn't even have a mother.*
The quote is untrue (which is why it's funny) and true (which is why it's sad). GLaDOS constructed the "you suck speech" with the aim of discouraging the test subject, and the writers constructed the speech to be comical.
EDIT: I'll echo Amadeus's answer, that the ranter, to be true to their character, could rant about what bothers them most about the other character. I'd also add that while the ranter may be annoyed and want to vent, their motives could lie elsewhere. Going back to your example, GLaDOS is probably annoyed that the test subject is misbehaving, but she's making up the "horrible" test results in order to influence the test subject.
This just makes the point that there can be many different kinds of "you suck" speeches and they'll have varying degrees of venting, discouraging, distraction, etc.
*Please forgive my limited knowledge of the Portal universe--if the test subject has parents, correct me.

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Such speeches are an emotionally cathartic release for the character making them, therefore the rule is they are generally in the order of what is irritating them most about the person that sucks.

The person making the speech is venting, and like venting, the highest pressure gases escape first, and they stop venting when most of their internal pressure has been released.

Like all such speeches, when read aloud, twenty or thirty seconds is acceptable and you push the limits of credibility at sixty seconds. They are meant to be burns; and shorter burns are better and funnier than long burns.

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