Bali
Diving Bali is something that should be on every diver's bucket list. Bali dive resorts are top notch, and very affordable when compared to many other dive destinations. We only dove in two locations in Bali - Nusa Penida and Tulamben - but both were world class dive destinations.
Nusa Penida is a small island off the Southeast corner of Bali, with reliably great manta encounters at Manta Point, and molas at Crystal Bay from June to October. We were there in April, which meant no mola encounters, but the water was very warm (comfy in a 3mm wetsuit), we had fantastic manta encounters, lots of turtles and octopus, and we had some phenomenal drift dives along pristine reefs.
Make sure you are relatively experienced before scuba diving here, as there are currents, sometimes quite strong ones, and they can be unpredictable. But as long as you go with a reputable outfit and are decently comfortable underwater, you will be fine. And be aware of the water temperatures - outside of mola season the water is quite warm, but in mola season it can drop as low as 12 or 13 C. That is really cold if you are in a wetsuit.
Tulamben is on the north coast of Bali, and is quite different of a destination. The water is warm and calm all year round. There are amazing muck diving sites, full of fantastic macro critters, especially nudibranchs of all shapes and sizes. There is the famous USS Liberty wreck dive, one of the most famous dives in the world. And there are great wall dives with huge sea fans, and beautiful reefs. Not the best place for really large stuff, other than some hawksbill turtles, and the bumphead parrotfish on the Liberty...but still fun for wide angle shooting.
The best time to dive Bali depends on what you are looking for. The rainy season runs from approximately October to April, with the dry season from May to September. Rainy season just means that a lot of days will have some rain, but it is often not too prolonged. The rainy season is the low season. We loved being there in the shoulder month of April because the weather was overall quite nice (with some rain, but not too bad), but most importantly, there were not a ton of tourists around. So, dive sites in Tulamben and Nusa Penida were quieter than peak season. October would also be nice, as it is a shoulder month but it is still mola time. The resort we stayed at in Nusa Lembongan recommended coming back for molas in October. Bali is the kind of place that makes you want to return to, and I think we will have to do that sometime soon!