I used to think that the underside of the feet was more deserving of being wiped than the top, until I saw the Messenger of Allah ﷺ wiping the top of them.
حدثنا وكيع حدثنا الأعمش عن أبي إسحاق عن عبد خير
كنت أرى أن باطن القدمين أحق بالمسح من ظاهرهما حتى رأيت رسول الله ﷺ يمسح ظاهرهما
Its chain of transmission is sahih. Abd Khayr is Ibn Yazid al-Khaywani al-Hamdani, a mukhadram taabii who is trustworthy. He lived past the age of 120. Al-Khaywani is a designation referring to Khaywan, with fatha on the kha, sukun on the ya, and fatha on the waw, which is a clan of Hamdan. See al-Lubab 1:401.This hadith is not found in the six books, and it is not mentioned in Majma al-Zawaid. However, Abu Dawud narrated a hadith with a similar meaning from Ali, "If religion were based on personal reasoning, the underside of the leather sock would be more deserving of wiping than its top, but I saw the Messenger of Allah ﷺ wipe the top of his leather socks." Daraqutni also narrated it. See al-Muntaqa 309, and see also what follows at 917, 918.