$20,887.51 budget, huh? Well, my first choice is obvious: The MkVI Golf ($18,240 after destination charge).
I can't vouch for the Ford Mustang (not only is it breaking budget by a few dollars, but) partly because every bloody accident that happens here in Los Angeles seems to have a Mustang super-wrecked on the shoulder at the front of the traffic jam.
I wouldn't buy a Honda Civic either. I've test driven an '06 and rode with my friend's sparkling '09 and there are so many things to complain about (the door handles are counter-intuitive, the dash is freakin' huge, the car has absolutely no soundproofing - worse than my '03 Golf, etc.).
A Mazda3 ($20,290 after destination charge) would be my second choice, even though I'm not too knowledgeable about Mazda. But nevermind, I suppose it's good that I have nothing to complain about them compared to other Japanese manufacturers like Honda and Toyota (who I think are cheap and self-serving, respectively).
My third choice would be... damn, do I have to pick one? Well, there are six to choose from from Nissan, so I guess it could be either of them.