Mu Ko Angthong National Marine Park, Surat Thani Province, Thailand.
It's quite a trek to the viewpoint at Ko Wua Talab, but the views out over Mu Ko Angthong National Marine park are worth the climb. It certainly doesn't feel worth it while you're scrambling over razor-sharp rocks that threaten to snap your ankles with every step. The punishing heat doesn't help matters much either. But none of that matters when you reach the pinnacle.
Ko Wua Talab sits more than 500m above sea level. So pack a decent pair of trainers, an extra memory card, and a bit of sweat equity. You're going to want to do this climb.