There are exceptionally good waves in several of Indonesia’s islands: Sumatra, Java, Bali, Lombok, Sumbawa, etc. But to find great surf you need to take into account two other major factors: winds and crowds.
To avoid windy conditions you either surf early in the morning (before 9 a.m.) or you go to a light winds region. But most of the Indonesian archipelago is exposed to the trade winds, blowing from the southeast from April to October, and from the northwest during the rest of the year.
The only region in Indonesia not affected by the trade winds is located at the so called Intertropical Convergence Zone (aka the Doldrums), a “belt” around the Earth spreading 2º north to 2º south of the Equator and characterized by light winds.
And the only Indonesian islands situated at the Doldrums and at the same time receiving the Indian Ocean super-consistent swells are the archipelagos of Mentawai, Telo, Nias, Hinako and Banyak. These small islands, in the hundreds, have very light winds for most of the year round, and at the same time they are directly exposed to the ground swells traveling all the way from the stormy seas down south the Roaring Forties, until they hit Indonesian coral reefs 3.000 miles (5.500 km) away, therefore with exceptionally long periods, sometimes over 20 seconds!
That’s why these little Sumatran islands have the best surf in Indonesia (and probably in the whole world!), with very consistent swells, lots of windless glassy days, and literally hundreds of surf spots.
Crowds are another problem pretty much everywhere there are good waves. This is happening in Bali, Lombok, Sumbawa, etc.
But Sumatran islands (Mentawai, Telo, Hinako, Banyak, etc.) are not accessible by car or airplane (there are no airports), and there are no places to sleep or eat near most of the surfing spots. To get there one needs a good boat (not cheap!) because these islands are 100 nautical miles (180 km) away from Sumatra big island. Therefore crowds are minimal there, and many surf spots are actually empty.
In short, Sumatran islands are the best choice to catch perfect and uncrowded waves due to a unique conjugation of factors:
They are the only islands directly exposed to Indian Ocean ground swells and, at the same time, located on a light winds region (the Doldrums);
Crowds are minimal due to difficult access and lack of tourist facilities;
Many surf spots are still unknown to most surfers.