I finaly figured out what the hardest water to fish is.  

Flash Flood water, when the water is dropping at a fast pace.  

First time I ever encountered fishing this water.  I could actually watch the water level drop in the lake while we were fishing.  In 5hrs before I was there the lake dropped about 18-24"s.  While i was there it dropped about 4"'s.  Nuts I tellya nuts.

Whats the hardest water you have ever fished?

Ice

I have to agree with Bassin_Fin@tic, but besides that, the hardest water I ever fished was just the opposite - a lake that was almost dry and then rose up out of it's banks in about a week's time.  There was emergent grass out 50' from the "new" shorline and the fish were buried in the grass.  If you were lucky enough to tap one on the head with a jig or heavy sinker, you might get a bite.

Bull Shoals, right after ice out.  You could see the bottom in 25 ft of water.  You could actually see fish way out ahead of you but when you moved your arm to cast they spooked.  

Club tourney.  One fish won, about 12", everyone else blanked.