You guys do realize that everybody has their own "best" anime.
I've watched tons of anime...more than the average person on this site i would say and i really like One Punch Man.
I mean most people who watch a lot of anime dont have a single favorite but a range of 10 - 20 top anime that they like and recommend to others.
One Punch Man is definitely high up on my list. Doesn't take itself too seriously...which is the great thing about it. Besides that, the anime just gives you a good feeling while watching, and the music and action really gets u pumped. I dont find it too cliche either, which is super rare across most anime.
But yeah...to see so many anime fans come in here and say "its clearly not the best" or "its only liked by people who dont watch anime" is disappointing.