I had a student ask me what the difference was and I choked
It seems as though the "of" is implied in On top. "Of all the groups, I'm on top." "The book is on top."
Not a grammar ninja
Life IS Goodł
"See the pile of books over there? The book you want is on top."
"The suitcases are stored away. The big one is on top of the wardrobe."
= on top of (something)