First, superlatives scare me almost as much as dive operators that jump on here with aggressive sales techniques! Second, there are a few tips that need to be stressed for divers who wish to encounter the mola molas:
1) Pick the right time of year. Season runs 6 weeks between end of July and begining of October. This slides back and forth every year. Out of season, you may still be lucky. However, luck is just that and no one can control it... not even THE BEST DIVE OPERATOR ON BALI! Off season, focus on full moon, or, even better IMHO, new moon and expect strong currents. In season, do the same, but also expect cold waters (see Tip 4).
2) Do multiple days of diving at the right sites. The right sites include Crystal Bay, Toyapakeh, PED, Sental, Buyuk, Gili Biaha, Tepekong & Mimpang. At times some are better than others. This can change from a couple of days to another with no notice. CHOOSE AN OP THAT ALLOWS YOU TO SWAP ACCORDINGLY.
3) Dive deep (and get training as needed). This is a deep sea fish after all. Meeting it should not happen when you're a coastal diver. It only does happen due to the Indonesian through-flow and its world famous upwellings (See Tip 4). My personal average sighting depth is 37m. I may dive a bit deeper and a tad longer than some. This is OK only as long as I can show adequate training, be it Deep, Nitrox, Deco or such with experience to match the diver card... The guys doing deep rebreather trimix dives in calmer parts of Bali get lucky all year round... at 65m-90m+!
4) Get ready for the cold. Everyone gets hit by cold water in a different way. I've had temperature amplitudes of 16C + (from 30C down to 14C) on a single 60 minute dive at Crystal. Take whatever you need, be it a 7mm, 5mm, 3mm long or just shorts, if, like me, you come with free blubber!
5) Remember that you can have a great dive without seeing a sunfish. Most of my best dives had no added sunfish! In my experience, the most single track-minded divers are also the least lucky. Call it Sod's Law!
6) Choose a safety conscious operator. More often than not, that's not the one who claims to be the best... Questions to ask include how many years have you been operating on Bali, do you operate your own boats or just rent any odd boat, how many years have your guides been diving these sites, do you bring oxygen on every trip, how about an Emergency Action Plan -> can I see it?
