What's the heaviest ball to hit, flat, spin or some of both?
Agassi seems to hit the ball quite hard with no spin, Nadal is hitting it hard with a lot of spin and Federer mixes it up a little more.
But which of the three hit balls is the toughest to handle?
The toughest ball to handle is the one your opponent is least suited to attack. It's usually advisable to play with variety -- do whatever you can to keep your opponent from correctly anticipating your shots. If you're familiar with your opponent's game, you also might tailor your approach to best match his weaknesses (and conversely to best avoid his preferences); in other words, if he hates playing high balls, use more topspin than you might normally use, but continue to vary your shot selection in order to keep him from getting into any sort of groove.
imo a heavy ball is one that has enough speed & spin to punish any even slight mishits. the key is with both forward momentum and rotation the ball stores a lot of energy. if you hit the opponent's heavy ball perfectly in the sweet spot it won't feel "heavy", but it is much harder to do. all pros hit with lots of spin not flat but agassi and some hit relatively flatter.
The way mentioned players hit, and any player for that matter, depends mostly on waht they're comfortable with. Grip, overall technique and style dictate what kind of shot will feel best. They all can variate the amount of spin and again perhaps some find that easier then others, but they still have a comfort zone of spin and trajectory.
Best way to find out is to try the sickest amount of spin you can hit and then some flat shots. Just play around and see which feels best. Learning to hit with spin was the best thing I ever did for my game. Now I feel a great deal more confident to play matches.
Wouldn't a ball feel heavier and heavier if, for every shot, hit at a somewhat constant pace and trajectory but with an increasing amount of spin?