残忍 (cán rěn) is the most direct and common word for 'cruel, ruthless' — it describes someone who deliberately causes pain or suffering. It is more general than 残酷 (cán kù, which can also mean 'harsh, grim' in situations) and 无情 (wú qíng, which focuses on lack of sympathy). For a stronger, more savage tone, use 凶残 (xiōng cán) or 凶狠 (xiōng hěn).