The ATS has excellent handling, but is too new to have much of a record.
The Camry and Optima are quick, but don't handle all that well and aren't especially fun to drive. Among midsize sedans, my pick would probably be the 2014+ Mazda6. After that, the Ford Fusion, with the 2011-2012 probably the most reliable.
But my personal preference tends to be for something smaller. The Acura TSX handles nicely with the four, not so well with the V6. An Infiniti G37 could also be a good way to go. The last time I checked nearly new ones were surprisingly affordable.