Naruto or Boku no Hero Academia, depending on how recent you mean by "modern".
posted over a year ago
From what I've noticed the majority of young people on the internet only consider stuff from post 2010 to be modern.I myself disagree with that notion though because to be everything from these current two decades of the 21st Century is modern.
I doubt it counts as "modern" any more, and there are other animes that probably top it, but since someone else put it, I have to promote what was one of my favorite if not my favorite cartoon as a kid
Secret Saturdays
Its only competition at the time was Teen Titans really XD Maybe Last Airbender
Loonatics Unleashed
Teen Titans
Pokémon
My Life As A Teenage Robot
Lucky Star
Secret Saturdays
Danny Phantom
Ed Edd n Eddy
Yu-Gi-Oh
Yu-Gi-Oh GX
Fairly Oddparents
Pop Team Epic (a new fave XD)