Stereotyping time! What's your least favorite "kind" of customer?
Today there must have been a business convention nearby, because there were buttloads of entitled middle aged jerks in businesswear, ordering 5 or 6 drinks at a time. Then in comes a group of six people who get 9 drinks, 4 bagels, and 3 sandwiches, all ordered one at a time, but on the same bill. Not only did they not tip, but my bar guy overheard them snickering that they "should have come through drive through, just to be more complicated".
This was a pretty extreme example, but entitled business yuppies who don't tip are possibly the worst.
Also middle aged women who sit in groups of 4 or 7 or 10 in the cafe and shriek and cackle at a completely unnecessary volume level.