What it means
Top banana is a jokey, a bit old-school way to mean the main person in charge, the big boss, the one calling the shots. It usually lands with a wink. You can use it to big someone up or to gently take the mick, and the tone is what makes it sing.
Usage examples
"The delivery bloke said the manager's not in. I went, Righto, where's the top banana then? Need this sorted before I lose it."
"Ask the top banana about the budget, the rest of us just push the buttons we are told to push."
"She started in the post room and twenty years later she is top banana of the whole company."
"I don't need the assistant manager, mate, I need the top banana who can actually sign this off."
"He walks in five minutes late every day acting like he's the top banana round here."
Where it comes from
It comes from American vaudeville in the early 1900s. In comedy double acts, the lead comic was the top banana and the supporting comic was the second banana. It then hopped out of showbiz and into everyday speech for whoever's running the show.
