depends on what you like. $20k is enough to buy a decent car. Ford Focus (SVT) is a good idea. Ford Escape is great if you want a bit more room, but I dont know if they can be had for $20 or not. VW Jetta/Golf are decent cars too. If you want a japanese car (not my taste) Toyota Celica is cool, but small. Honda Civics are okay, but everybody has one, and I dont think they are worth what they cost. If you live where the weather is nice Cameros and Mustangs are fun fun fun cars. Insurance could be high. I have to say the Focus SVT would be great. Its quick, and should be reliable. I think they are $18k. And you'll get a warranty for 3 years, so in 3 years you can buy another car.
-Alex