The phrase "reigning champions" means the team or person who is currently holding the title of being the best in a particular sport or competition. Full definition
Last year's reigning champion returned and once more came out the winner. (driving.ca)
Mourinho already knows Fabinho from his time in Spain with the current reigning champions of Europe, Real Madrid. (beyondtheposts.net)
Being the current reigning champion in the 2006 scovie awards for wing sauces, the masses are flocking to this concoction, and singing the praises of the defcon creator for reaching above his standard perfection they have all come to know him for. (hotsauceblog.com)