Nusa penida, nusa lembongan or - ceningan also know by some as Bali's Jurassic park. This is (obviously) because we can find the bigger creatures of our ocean here. Almost the whole year round you have a good shot, roughly 90%, at seeing the Reef mantas. From August until late September, it's when the oceanic sunfish comes up for its annual cleaning.
Either you can stay on one of the islands or do day trips out of Bali. There are two harbors, Padangbai and Sanur, from where speedboats or ferries depart to Nusa Penida. Crossing to the island normally takes 45 minutes with a speedboat or expect a long waiting time + 2h boat trip with one of the older ferries. The furthest dive site is where the reef mantas are. It's a somewhat protected bay against steep cliffs. So there never really is any current, sometimes some swell when it's low tide... Just be careful with all the boat traffic when diving at this particular dive site.
The same goes for the sites where the sunfish comes up. These are almost always sites with lots of currents that can be very unpredictable. Experience with current is recommended.