Mt. Dake, sometimes called Oshima-Fuji for its shape, is the island's classic viewpoint. From the terrace near the summit you get the best quick read on Suo-Oshima: ridges, citrus slopes, fishing villages, and the small islands scattered through the Seto Inland Sea.
The road reaches close to the top, and the site also draws hikers and paragliders. On a clear day, go early or late enough for softer light; the view is the point.