I have done a lot of shopping in that price range recently (as in the last week)...
How much people and cargo hauling room do you need?
How many amenities are you looking for (BT, Satellite Radio, Leather, Heated Seats, etc, etc.)
What kind of MPG are you looking for?
What kind of sportiness performance are you looking for?
What kind of resale are you looking for?
Depending on your wants/needs the "best bang for the buck" (in my opinion) in that price range new in no particular order:
Honda Civic
Toyota Corolla
Honda Fit
Ford Fiesta (many people do NOT like the feel of the dual-clutch auto transmission)
Ford Focus (many people do NOT like the feel of the dual-clutch auto transmission)
Chevy Sonic
Chevy Cruze
Hyundai Accent
Hyundai Elantra
Kia Rio
Kia Forte
Ka Soul
If you look at slightly used (1-3 year old lease turn ins etc. with low mileage), your options grow exponentially. You can find well equipped mid-size sedans (Accord, Camry, Fusion, Mazda6, etc) for @$20K, and the cars listed above for thousands less than new.