It's free, easily available, readily playable without having to change your system locale or use appLocale, and it's mostly decent.
Also, sentimentality. Seems a lot of people started with it, so people tend to recommend it.
I wanna say Ping Pong the Animation.
Its art style will let it remain timeless, it's largely unconventional, and since it's not simply just a sports anime it will have wider appeal I think.
Also, Gurren Lagann is still within the last decade, so that.
I mean, it's so overused that it's become almost a non-word by now. Which is amusing, since in the comic it originated from it doesn't have any meaning to begin with.