2. Lemons and limes
"The pH of straight lemons is 2. If you suck on lemons or limes with your front teeth, the enamel comes off the front surface of the tooth. They'll look really yellow and get sensitive."
1. Whatever you just ate, on the way back up
"The worst thing you can do is vomit or have acid reflux. It's so bad that we can usually tell if someone's bulimic just from looking. We'd never want that to keep someone from coming to the dentist, but it erodes the insides of people's teeth in a [specific] pattern. The only way to do so much damage is from refluxing or vomiting all the time. Since that food is already digested, you're essentially giving your teeth a pure acid wash."
Dan Gentile is a staff writer on Thrillist's National Food and Drink team. He was totally lying earlier about flossing, but promises to do better. Follow him to a cavity-free smile at @Dannosphere.