Even though I love Bali, I would definitely recommend SIPADEN ISLAND (east Malaysia/Borneo). Sipaden rocks because you get to see the big action such as schools of chevron barracuda, turtles, rays, schools of bumphead parrot fish, and sharks, and can also see the macro stuff like nudibranchs, pom-pom crabs, etc. Sipaden Island is a marine reserve so you won't be able to stay on the island itself, but you can either stay at a near by resort or do it by liveaboard. I would recommend the liveaboard experience for Sipaden.