Telemarketers, because they invade your personal space in your home, usually at the most innoportune times like dinner hour, or when you're getting ready to leave for an important appointment.
We have our number on the national "do not call" registry. So, when they call, I just cut them off, tell them that, and remind them that it's a $50 fine for each call after being notified. Then I hang up.
Used car salesmen don't come knocking at your door--it's YOUR CHOICE to visit their place of business, so you either learn how to deal with their shenanigans, or do without their wares.