nobu is excellent and best scenery, make reservations pretty far in advance. shibuya in MGM Grand also very good, but more quiet scene if that is what you are looking for. i've wanted to try social house at treasure island, i hear good things, but haven't been there yet.
We ate at Sushi Roku when we were there back in April. Its at the end of the Forum shops on the top story, with good views down the strip if you get a window seat. I haven't been to Nobu so can't say which is better, but the Toro was excellent here, as was the rest of the fish.