second to none; unrivaled
A somewhat formal or literary set phrase meaning that someone is not inferior to others in a particular area. Often used as ～に人後に落ちない or ～で人後に落ちない.
He is second to none when it comes to effort.
I am confident that I am second to none in knowledge of this field.
The phrase combines 人後 (behind others) with 落ちない (not fall), literally meaning 'not falling behind others'. The exact historical derivation is uncertain, but it is a classical Chinese-derived expression that became a set phrase in Japanese.