Don't know about the best, but...
Atlanta, overall probably has the best freeway system for my money along the following dimensions...(1) the roads are very smooth, generally asphalt as opposed to concrete, which can be bumpier (I believe I saw a ranking last year or earlier this year from the Texas Highway people ranking Georgia's roads as the best, Michigan and Mass as the worst); (2) there are not a lot of surprises... not lanes suddendly dropping away, short entrance ramps, or peculiar curves; (3). aesthetically pleasing... the soft hills and curves, the trees lining the freeways, landscaping generally... It all flows and actually makes for a rather pleasant driving experience. There is a lot of traffic in Atlanta, but it really does move quite briskly. It is amazing how quickly you can get from the airport on I-85, through downtown on the Connector, and to Buckhead on 400. The Kennedy in Chicago, the 405 or the Santa Monica in LA can literally break your back. THe Connector in Atlanta is equally as busy as each of the abovementioned... just moves better... Its all about the design.
My biggest issue with Atlanta's freeway's is NO lights on 285. What is Georgia DOT thinking... or not? Can't say much about the major street system other than the geography does not lend itselt particularly well to a grid... and the tendency for crusining on Peachtree on the weekends seems small townish.
Dallas
Like Atlanta, you can generally move briskly through a lot of traffic. 75 has been reworked... and it works... however, 35 doesn't. Not enough lanes on 35 coming in from Denton...very narrow shoulders... and then that "Mix-master" (which my sister just carries on and on about) is so poorly done (engineered) until it is about as bad as the junction of the Eisenhower, the Kennedy, the Dan Ryan, and the COngress downtown Chicago. An accident there can stall the entire freeway system. For someone who doesn't drive on 35 everyday, coming in from Denton and getting to that intersection can be trumatic. The singage is not particularly good so you have to make a real quick decision about how to follow 35 south to the Thornton... the sign is under a bridge or something, and the lanes don't follow the logic that you might expect. Overall the Dallas system is disappointing for what one might expect to be a pretty sophisticated system given Dallas' braggadiccio.
Nashville... = disaster. Short ramps, odd curves, constant road work...Why can't 24 and 65 ever get completed.. Drives me crazy...
Miami,,, basically, just one very busy north-south artery, I -95