Naniwa Professional Stones
These are widely considered to be the finest artificial whetstones available today. These new Pro Stones are the next generation of the highly-regarded Chosera stones, which they replace. They are made by the magnesia bonding method and so are very hard, very fine and smooth and do NOT require soaking -- just spray/splash and you're good to go. Very easy sharpening, very fast cutting, and long-lasting. They have a high ratio of abrasive material so they do indeed cut faster than many other stones. These stones are great for the pro or serious home use, as they do not have bases and are a full 2cm, so they'll be very long-lasting. As with the Super Stones, these are NOT attached to a base, as requested by most sharpening and cooking professionals... this way they are easier to store and carry, and you have more choice in your sharpening setup and you do need a base, you can purchase the steel/rubber base sold separately here which provides MUCH better support and stability.
One thing to keep in mind: the Chosera/Professional stones are more fragile than cheaper stones, so to keep them from cracking it's important to dry them slowly and naturally (do NOT use an oven or dryer or anything like that). And when wetting, use ROOM temperature water to avoid rapid and extreme temperature changes. Then when you're done just dry them with a towel, and set them aside to dry at room temperature. Also please do NOT soak these stones -- soaking may cause cracks to develop.