Bluefin tuna is the Kobe beef of tuna, and the o-toro cut (fattiest) is like the rib eye of the fish. It should be enjoyed either raw or very rare with minimal flavoring agents that will detract from it's natural oils and flavors. It is a delicacy and one of the most prized of all types of fish. Because of that, it is also subject to overfishing and due to the fish's slow rate of growth, the bluefin population has declined severely which in turn has caused the prices to increase. In public fishmarkets, you may find the tail/back and loin cuts available and possibly the back end of the belly. The cuts of the belly closest to the gills (o-toro and ju-toro) are always sold premium to restaurants.
"Boldness has genius, power, and magic in it."
Johann Wolfgang Von Goethe