I can also recommend Sansui, which is on Rue de la Faucille, just off rue Fort-Barreau as mentioned by LJM. I've eaten there a couple of times so far, and found it excellent and the staff very friendly, although I admit not to being a sushi expert (yet).

Conveniently for me it is right under my apartment but if you are travelling, it is about a 2 minute walk out of the back of Geneva station.