Adverb
Used as an intensifier with superlative forms of adjectives.
Best of all was the bicycle I got. First of all, I'd like to thank my agent. He was the greatest playwright of all.
Used as an intensifier with nouns to denote being ultimate.
Source: en.wiktionary.orgCare most of all for your soul, brother, it is your only treasure. Everything else you own, doesn't really belong to you. Nikolaj Velimirovic
Ignorance, the root and stem of all evil. Plato
Experience is the teacher of all things. Julius Caesar
Thinking well is wise; planning well, wiser; but doing well is the wisest and best of all. Persian Proverb
Medicine is the noblest of all arts. Latin Proverb
Money is the root of all evil. English Proverb