Shapton Pro 8000 or higher is what first comes to mind. Naniwa Professional 5000 is a very nice stone, and fairly hard for a water stone. The 10k is very soft, so is not what you are looking for. The Naniwa Lilly White 8k is a great finisher. Also as many have said, the Spyderco ceramics are about as hard as you will find.

I try to base my reply on my experience, and this is what I have experience with. I always say, “If it gets your tools sharp enough it was the right stone for you.”