Another vote for Jack's Diving Locker in Kona. I had an AMAZING manta dive about a year ago. The mantas first showed up when we switched the boat lights on at sunset during a surface interval after an afternoon dive. Of course, we immediately hopped in with our snorkels! About four mantas were hanging by the boat, and for some reason they like to flip somersaults right beneath you if you're snorkeling on the surface. After about 1/2 an hour of swimming around, we got back on the boat, donned our scuba gear, and headed over to where a French documentary team had rigged up an incredible light system. Eight big mantas were spiraling about in the cone of light; we sat on the bottom and just stared up in amazement. When the French team left, we turned on our flashlights, and the mantas swooped down and did pivot turns practically on our heads. It was unforgettable.
I went back about 6 months later to try again, and not one manta appeared. But don't get me wrong -- a night dive in Kona is NEVER a waste! That was a spectacular dive in its own right...