The Sultan Omar Ali Saifuddien Mosque, a landmark in Bandar Seri Begawan, the capital of Brunei, is one of the most beautiful mosques in Southeast Asia.
Named after the 28th Sultan (father of the current Sultan), it was completed in 1958. Blending Mughal Islamic and Italian styles, it is built on an artificial lagoon. Photos I saw showed it was especially beautiful when the water was on, but unfortunately, it was dry when I visited, so it wasn't quite as stunning.
Admission is free. Non-Muslims are only allowed entry during designated times. Appropriate attire is required; robes can be borrowed free of charge. Slippers are necessary. The lights at night, reflected in the lake, create beautiful photos. Opposite is the popular photo frame; be sure to take pictures there.