If you're looking for great sushi in Park Slope, there's really none better than the Gingko Leaf on Union between 6th and 7th avenues. The seafood is very high quality, the dishes are interesting and the staff is exceptionally attentive. The restuarant seems to still be fairly unrecognized by the greater Slope dining populous, possibly because it's off of the main avenue and has a store in the front featuring specialty Japanese housewares. Luckily, for those in the know, there's never a wait for a spot at the small sushi bar or a quiet table in their lovely bamboo enclosure with lily and koi ponds.
I was just there Friday night, where specials included butterfly fish sashimi (which I'd never seen - succulent and sweet), big eye tuna sashimi, kyoto style eel sashimi, grilled chive with bacon...just to name a few that I ordered. My favorites on the regular menu are the yellowtail, blue toro (fatty tuna), and monkfish liver, which is prepared particularly well here. From customary fare to more innovative items, everything on their menu is creative and well-conceived, with an obvious attention to detail anda balance of flavors. It far surpasses the neighboring sushi options.
Highly recommended. Not to mention very reasonably priced.