In Jain Dharma, samayik is generally regarded as the greatest among these for reducing bad karmas, because it creates equanimity, inner stillness, and detachment — the very heart of Jain साधना.

Why samayik is highest:

• It directly शांत करता है raag-dwesh
• It helps in stopping new karma
• It supports true spiritual purity

Upvas and brahmacharya are also very powerful austerities, but if you want only one, then samayik is the best answer.