The DMT W8F 8-Inch Diamond Whetstone Sharpener is my favorite. You use it with water so clean up is easy, and the one stone lets you get a knife sharp enough to shave hairs off your arm. And it takes about 10% of the effort it takes to get the same sharpness using medium then hard Arkansas stones with oil.
DMT W8F 8-Inch Diamond Whetstone Sharpener, Fine with Hardwood Box - Sharpening Stones - Amazon.com@@AMEPARAM@@http://ecx.images-amazon.com/images/I/41J6V91288L.@@AMEPARAM@@41J6V91288L