Seesharky,
I don't want to get into recommendations about who to dive with in Bali except to warn you to be very cautious of people confidently recommending the "best" operator in Bali "hands-down!" - someone who happens to be the only operator they ever used on their single visit to the island. But enough about my peeves. I can tell you that the best time is basically now - most people probably would say the mola season in Bali starts in July (or perhaps earlier) and extends through September, perhaps through October.
But I need to warn you that the number of boats taking divers to see molas at Nusa Penida is reaching plague proportions. I happened to visit BIDP (a Bali dive operator) today, where I spoke with one of the dive guides. He told me they had just seen seven mola molas at Crystal Bay. Unfortunately, there were twenty (!) dive boats there at the same time, each with as many as 10 or even 12 divers. If you don't already know, there are only two moorings for dive boats in Crystal Bay. Some of the other boats tied on to boats already on the moorings, but others were dropping anchors, including onto the coral areas as well as the sandy parts of the bay. And today is a Monday - apparently the weekend was worse.
Earlier today, Marthen Welly, the new coordinator for marine conservation programs at Nusa Penida for The Nature Conservancy, told me there had been fourteen boats in Crystal Bay at the same time one day last week. Marthen also said that operators are reporting that some divers are harassing molas by approaching too closely, touching, etc. This is not good at all.
Of course it is good for Bali that there are more visitor arrivals this year. Unfortunately, however, Nusa Penida is not currently a protected area, and there is no legal mechanism by which the number of boats or divers at Crystal Bay or other mola cleaning sites can be controlled. Although some far-sighted operators have adopted a code of conduct for diving around molas, not all the operators are distributing copies or requiring their clients to observe the rules. The upshot is that Nusa Penida is becoming a sort of free-for-all in which it is the molas are the ones most at risk.
Sadly, we won't know what the ultimate limit in terms of how much harassment and excessive contact molas can tolerate is until it is already too late. Please keep this in mind when you plan your trip. There is a real danger that your chances of getting a good filming opportunity will be compromised by the swarms of other divers in the water, and there is also a risk that the number of molas coming to Nusa Penida reefs in months and years to come may be negatively affected by the excessive numbers of divers crowding around these strange and wonderful animals. You might have better odds of getting a good filming opportunity early or late in the season, when the number of fish coming in for cleaning may be fewer, but the number of divers in the water will also be less than at the height of the season, which is right now.
Frogfish