唯一 (wéiyī) is the standard adjective for 'only, sole' — it directly modifies a noun with 的. It's more common and neutral than 单一 (dānyī), which often implies 'single, uniform' (e.g., 单一经济 'single-product economy'). 单 (dān) as an adjective means 'single' (not paired), not 'only'; 独 (dú) is an adverb meaning 'alone/by oneself'.