Mori Ohta Sushi, in Jardins neighborhood, is awarded every year as the best all-you-can-eat Japanese restaurant. It's really good.
Every time I go there I eat something different, but one of my favorites is a roll made with salmon, shrimp and roe - I don't know how it's called, but it's delicious to have it with some white wine! A nice order too is the lychee caipirinha. The tuna with foie gras is also a great item on their menu.
It could be just a regular Japanese restaurant, with a kinda hip decoration and hipster habitués, but I have a little secret for you, which makes it exquisite: ask for one of the few spots at the bar, not in the tables, because there the sushiman makes you a special meal, prepared just for you, with a variety of food that you won't get by going there as a regular customer.
Everyone goes for the tables, so be ahead of them!
You must be aware of the waiting list, which is always a long one. Be ready to wait like an hour.