Why I picked One Piece. 

I picked this anime for our first input because of its extremely simple vocab, super casual dialog, repetitive vocab choices, and high frequency of highly common words in rapid fashion. Out of all the One Piece arcs, this one is especially high in the qualities we are looking for. (I've done tons of word frequency analyses on One Piece.)

It also has arguably one of the best stories in the entire series. Making it extremely "emotionally sticky", meaning the Japanese you learned sticks in your mind longer.

And last but not least, One Piece is the MOST watched/read and most popular anime in the history of Anime in Japan. It's crazy huge, way bigger than it is in America, WAY BIGGER. Meaning it has a massive impact on the actual culture of Japan, believe it or not. Even Japanese people who don't like anime or manga in Japan, have watched One Piece at least once and accept it as HIGHLY influential.

Despite your stance on the series, it HAS influenced the culture massively, and for our purposes, THE LANGUAGE.

(Pro Tip: Highly popular TV shows and movies, greatly influence, and for that matter, change, the language of a country. America is the same.) Making this series extremely valuable to us language learners.