of all

adverb3

1. •
   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.
2. •
   Used as an intensifier with nouns to denote being ultimate.
3. •
   Marks a thing or person as a particularly unexpected or unwanted example of its category.

   • While on holiday in Spain, we ran into Mark, of all people.