Story:
One Piece has one of the greatest story's I have personally ever read, and a large part of that has to do with the completely unique story One Piece portrays. Yea, you could say that One Piece has elements from other things, but doesn't everything? The story is just grand (outside fillers, which surprisingly, aren't even that awful. And Skypia. I think I'm the only one that doesn't like skypia though) and when things happen, even to characters that you know won't be around during a majority of the adventure, you tend to feel for them.
Art:
Unlike most older animes, One Piece's art style has honestly aged well, and only gotten better. Nuff Said.
Sound:
The OST for One Piece is honestly One of the best Piece's of music I've ever listened too. (get it?) The openings are fantastic, and so are the endings.
Characters:
The characters in One Piece are extremely unique, and are pretty much all memorable, if not by name, but by what they looked like or what they said. Some characters just had a certain way of speaking, others looked extremely funny, and some were just memorable in general.
Enjoyment:
Some animes that are much shorter than One Piece have often gone on the backburner when I get in the mood for One Piece, mainly because if I'm in that mood, I can't really scratch my anime itch unless I watch some One Piece.
Overall:
Definitely the best anime I've ever seen, and I challenge anyone to give me one better.
