Debt where you have nothing to show for it.
Car loans? At least you have a car. Mortgage? You sleep in what it bought every night.
People with credit card debt often have no idea how they got there. Or if you got a car repossessed or a house foreclosed, but still somehow owe the debt (typically because they sold the house or car for less than what you owe and expect you to somehow pay the difference). It’s like you owe someone money for nothing.
Thankfully I don’t have this kind of debt.
The second most annoying debt is debt because you overpaid something. Rent-to-own places are notorious for this. New car dealerships are too, but I don’t think it’s as bad as rent-to-own. And student loans - it’s so easy to take out more than you need because there is no verification that you’re spending it on eligible items.
Unfortunately, I did take out too much in student loans. I did at one point have a new car loan, but that’s been paid for almost six years now.