I'm partial to the mid 80's to 90's rigid mountain bikes like this one;
http://gainesville.craigslist.org/bik/4249679821.html. This one might be a little on the small side but test ride it anyway.
Also check out some local bike shops and see if they sell used. They may be a little on the expensive side, but you can almost be guaranteed it's in very good working condition. Your area may also have bike co-ops as well.