The top banana is the most powerful or important person around. If you are promoted to top banana at work, it means you are the boss.
Besides top banana, there are a lot of other figurative ways to describe the number one person in a group, including top dog, big cheese, and and head honcho. Top banana has the distinction of having started in burlesque theater, though experts disagree about who used the term first. Some performers used bananas in their acts, and others used either top banana or first banana to refer to the headliner, the star of the show.