Michael Swan* states:
"After superlatives, we do not usually use of with a singular word referring to a place or group.
I'm the happiest man in the world. (NOT...
She's the fastestplayer in the team. (NOT...
But ofcan be used before plurals and before lot.
She's the fastest player of them all.
He's the best of the lot...."
*Practical English Usage, Third Edition, by Michael Swan. Oxford 2005