2006-2011 Toyota Prius
The original-recipe Prius has a lot more competitors out there these days -- including a doppelganger in the Ford C-Max -- but none of them are as good at their jobs as the car that started it all. Even the newer Prius v wagon can't touch its older sibling's 50 miles per gallon, while the Prius c compact provides the same mileage in less space.
Its 21.6 cubic feet of cargo space isn't the largest in its class and its 98 horsepower is easily the pokiest among hybrids, but those aren't the numbers that matter to hybrid buyers. Even newer hybrids have a tough time matching its fuel economy. If you're willing to roll the dice on its battery life, a used Prius can be a sound bet.