What's the worst insult I can call someone in Turkish?

Hey everyone, for context im born in Istanbul, in English speaking country since age 6 and trying to re-learn turkish to become fluent.

I hate one of my roommates and don't know how to refer to him when talking to my parents. I know that the direct translation of "Retard" is "Gerizekalı" but I want something that's a bit more insulting to his intellect.

Thanks!