What does Ravi think it would feel like to be "the winner in a circle of older, bigger, luckier children"?

In a word: awesome. Ravi wants to win the hide and seek game so badly that he can practically taste the victory.  

What fun if they were all found and caught—he alone left unconquered! He had never known that sensation. Nothing more wonderful had ever happened to him. . . 

Based on the above line of text, the reader can sense that Ravi sees his victory as winning a battle of some kind.  He likens himself to a conquering hero.  He alone would be victorious over his hide and seek "enemy."  

To defeat Raghu—that hirsute, hoarse-voiced football champion—and to be the winner in a circle of older, bigger, luckier children—that would be thrilling beyond imagination.

Ravi's enemy is Raghu.  Raghu is a bit of a bully and can't stand losing.  He's the leader who dominates the decisions of all of the other children.  Ravi admits that being able to essentially say "in your face" to Raghu would be more exciting than anything that he has ever imagined before.  For Ravi, winning, and beating Ravi, would be the pinnacle of his existence.