To really roast someone in a way that gets everyone in the room groan-laughing, you've got to hit them where it hurts, and that involves getting personal.
It could be going after something that they're doing in the room, some way back childhood fear, or just something you know about their personality or attitude that gets to them. So if you truly know the person you're roasting, you could probably come up with some better insults about them than anyone else.
Which is why no one could roast you better than your Mom. Usually though, she's too busy raising you and making sure you don't kill yourself to take the time to put you on blast. But some people are just born savage, and becoming a Mom doesn't change that.