They are all pretty good. It's like asking which is better, an apple, orange or banana. ^_^
But in my opinion (i have driven them all), here are what i noticed:
Nissan Altima is just a good balanced car. A/C is really freezing and parts are reasonable.
Honda Accord is a good sports car. Speed is really fast and durable but parts are quite expensive. The body is also more fragile.
Mazda 6 is a good car for maneuvers. The turns really dig in and is also speedy, but not as fast as a Honda. Parts are cheapest of them all.
Overall, it depends on your preference. Sporty, cheap or balanced of both worlds. ^_^
