ما هو أصح حديث في كل الأحاديث؟

The authenticity of a hadith means trusting its attribution to the Prophet, peace and blessings be upon him, by examining its isnad (chain of transmission) and matn (text) to verify the presence of the conditions for authenticity. This can be definitive and certain in mutawatir hadiths, which convey sure knowledge and whose authenticity does not vary, and it is not correct to say that some of them are more authentic than others.

As for ahad hadiths (solitary narrations), they include both acceptable and rejected ones, and their use as evidence depends on researching the circumstances of their narrators. Scholars have differed in designating the most authentic isnads, and many researchers have refrained from ruling that a specific isnad is absolutely the most authentic, because the varying degrees of authenticity depend on the isnad's fulfillment of the conditions for authenticity, and it is rare to find the highest degrees of acceptance in every single narrator of an isnad.