What parts of tuna are suitable for Sashimi?
There are mainly three types of Tuna(Maguro) served at Sushi restaurant: Yellowfin, Bigeye and Blue Fin.
The main difference is the size. The bigger the tuna, the more "tuna" flavor it has (generally speaking). Let's start with Yellowfin and Bigeye.
What's marked in green is suitable for Sashimi. Tuna uses its tail to swim, so the tail meat is the leanest, containing the least amount of fat, therefore tastes less flavorful compared to the belly. Here is a side view of Yellowfin tuna.
In the bottom right part, you see white tendon, which is too tough to chew. Because Yellowfin does not grow big enough to store fat as Blue Fin does, this part is unsuitable for either Sashimi nor Nigiri. It, however, can be used for Spicy Tuna after scraping. The best part is the top part (pointy triangular shape) meat. It is tender and has a very mild tuna flavor. Great for Sashimi! Thing is a bit different for Blue Fin. If you look at the whole fish, tail meat would be unsuitable for Sashimi just like Yellowfin.
However, because the belly meat contains a lot of fat, generally known as Toro, you can pretty much eat any part of Blue Fin Tuna as Sashimi, as marked below in green line.