For my knives, I use arkansas oil stones. I don't use synthetic stones, because after a time, they end up with a groove in the middle. And they don't usually have a fine enough grit for a polished edge.
You can also use a japanese water stone, which is EXTREMELY good for getting a razor sharp edge. But they're hellishly expensive.
Diamond stones I don't have any experience with, but I'm sure there are some members on here who use them.
Butcher's steels don't actually sharpen a knive. They simply re-align the edge bevel for a time, but after heavy use it is necessary to use a stone.
Grits are another matter. Different grits on different stones don't compare. Japanese stones use a different grit rating than arkansas stones, for example.
Adam