DCP, I've eaten a lot of sushi and seen it made hundreds of times. I could not have bested your effort.
Bulldogger, try quail egg yolks on top of you ikura (salmon egg).
Bluefin tuna is the most widely preferred. Yellowfin comes in second and is the most widely used.
Yellowtail is pretty good, but gets quite fishy if it's not absolutely fresh.
Salmon is good, but has to be "sashimi grade" or you could get a gut full of parasites; and, must be real fresh or it's too fishy.
Unagi (eel) is good, but I won't touch it raw.