Cab Drivers
If someone is a great navigator, hears your destination, has no followup questions, makes not a peep, elicits not a smell, you owe this white whale of drivers a minimum 20% tip.
For the rest of the 99% of drivers you'll likely encounter, 15% is the acceptable number to keep you well within propriety, and to save you from being yelled at.
[Photo via @appleortravel]