Obviously "best" is a little subjective, but it got you to click, didn't it? I honestly challenge you to find an animated series that has a better combination of humor, action, drama and great characters alongside fantastic worldbuilding and even better voice work. Trick question, because that task is impossible. I used to spend endless hours watching this with my brothers, loving all the cool fight scenes while wishing we had an uncle like Uncle Iroh (who is voiced by the legend himself Mako (Major RIP)).
If you've never seen it before, I promise this will be a great quarantine stream for you to start. You might be thinking "Oh it's a kids cartoon" or "I don't like anime!!!". To you, I say this: Well ACTUALLY It's not an anime, it's a cartoon.
And while it is a kids cartoon, it's really complex and deep. This isn't Total Drama Island or Peppa Pig or whatever, it's a really good show that happens to be a kids cartoon. Give it a watch with an open mind and I promise you won't be disappointed.