There are quite a few fish which are very good as sashimi. The most common ones are tuna, salmon (not Australian Salmon), kingfish, mackeral, silver trevally and even flathead (according to Tetsuya). Squid i believe can also be eaten in this style. I'd have to say my favourite is probably kingfish with a bit of soy and lemon, closely followed by tuna, with mackeral in 3rd place.