Where to find the best sashimi? Certainly Japan and nowhere else. First there's the quality of the catch around the Japan seas. Then there're the traditional way and care the Japanese fishermen and fishmongers handles the fish, unmatchable by foreign fishermen. You see, the preparation of seafood to be served begins here, from the sea to the market, not in the kitchen.
Then, there's the sashimi chef. Now the task is not cooking. It's simply preparation and presentation to serve. When you're slicing up a fish, one hand is holding the fish steady and the other hand is holding the knife to slice it up. For sashimi serving, every second that your fingers remains holding the fish is transferring heat thereon and therefore lowering its quality. To be fast but not mess up, the skills of the chef and the calibre of the knife are paramount. Such are seldom observed or trained outside Japan. That's why authentic Japanese restaurants outside Japan usually have at least a head chef transferred from Japan. That's also why the sashimi piece of raw fish always tastes better than the one on a sushi, because in making a piece of sushi, the whole palm is involved, i.e. heat transferred and quality lowered.
Then of course, there's the quality of the soy sauce and wasabi.
not forgetting different season served different fish ...make sure you ask the restaurants staff what is the best fish to eat in that season . here are some example:
Akoudai, sometimes called Akou or Menuke in some parts of Japan, has a very unique look because its eyes, which apppear to be almost popping out of its head. In English it is known as Red Rockfish, and as the name implies, its entire body has a beautiful bright red skin. They live in the deep sea of southern coasts of Japan, ranging from 500 to 1000 meters (1700 to 3300 feet) and this is the reason why their eyes are so extruded; because of the water pressure. The meat is identical to Kinmedai (Golden Eye snapper), containing high amount of fat and is extremely tender. It is suited for any kind of cooking method. The season for this fish is winter.
Kamasu
There are many kinds of Kamasu (in English known as Japanese Whiting or Baracuda) that can be caught on the southern coastline of Japan: Aka-Kamasu (Red barracuda), Yamato-Kamasu (Japanese barracuda), Ao-Kamasu(Blue barracuda) and Oni-Kamasu (Devil baraccuda), to name a few. The most popular variety is Aka-Kamasu, also known as Hon-Kamasu (True-baraccuda). Kamasu is a very aggressive predator that can be swim as fast as 150km/h (90m/h). It has slightly longer under jaw than upper jaw which has very sharp teeth in order to catch other edible fish. It can be grow as large as 50cm (16 inch) and the season is in summer and fall. The most popular cooking method for this fish is salt grilling, but it is also very tasty as sashimi when it's fresh. At some restaurant, they marinate with "Shiraita-Konbu" (white kelp) then then blow torch the skin so that customer can taste the smoky skin with tasty meat together.
Kisu
Kisu, Japanese sillago or whiting, can be found anywhere on the coastline of Japan except the Okinawa islands. It is also abundant from the south China sea to the Korean peninsula. It is impossible to talk about "Edomae-Tempura" (Tokyo-style tempura) without this fish because Kisu is very delicious when it is cooked with oil. Although Kisu is famous for Tempura, it is also suitable for most other types of cooking because of the it's delicate white meat. It is a very popular fish for anglers because you do not need any special skill to catch this fish. The best season for this fish is in summer.
Kuromutsu
Kuromutsu (Japanese blue fish) is abundant throughout the coastline of Japan, the southern part of Korea and the northern part of Taiwan. They love to eat small fish, shrimp, crab and squid; for this they have numerous sharp canine teeth on the upper and bottom jaws. Young Koromutsu live close to the shoreline but they change their field of activity as they grow up to the deeper ocean. As a matter of fact, most Koromutsu that we find at the fish market have been caught in the deep sea, ranging from 200 to 700 meters (650 to 2300 feet). Kuromutsu might not be the most appealing-looking fish, but it has some of the most deliciously marbled white meat in spite of it's appearance. You can find Kuromutsu almost a year round in Japan but the best season for this fish is in winter. The popular cooking method is poaching and eating raw as sashimi.
Bonito
The Bonito is a very delicious fish and it looks almost exactly like a baby tuna. However, it is not in fact a baby tuna and taste quite different from it. They usually live in warm water and comes up to the coastline during the spring in Japan. In New York, this fish's season starts around March and ends around October. It tends to get more oilier towards the end of the season.