Toyota Corolla. The cobalt is garbage. I have herd too many problems with them. Look at used car listings, why do you think there are so many more cobalts than corolla's, and the cobalts are cheaper?
I own 1989 Geo Prizm (It's a rebadged Toyota Corolla that general motors sells, like how the Toyota Matrix and Pontiac Vibe are the same car) that has 250,000. Although the car has seen better days and we have slacked in it's maintenance in the past 5 years and is rusted from living in a snowy region, (It's mostly used for round town driving now) it has not had any major breakdowns EVER! (We're waiting till it does before we get rid of it.)
Don't look past other brands either. I would also highly recommend Honda Civic which my family has had great experiences with as well. Now if you can get a great deal on the cobalt, then go for it, just like how I got a brand new Kia Spectra SX fully loaded for $10,699 (bought February 2009, car was a 2008 model with manual transmission) and it has better specs than a $19,000 Corolla. But if there is very little price difference, and you don't know much about repairing/maintaining a car, the corolla is a lot better.