I know, I know, One Piece is one of the "Big Three" when it comes to manga, so shame on me for posting a review about it, but I just LOVE One Piece.
The story is obscure, I agree, but what anime isn't obscure? What starts off as a simple adventure to sail around different islands, has begun to expand to something much more, with a LARGE amount characters, with a lot of thought and care taken into each and ever single one.
The artwork, near the start of the series, is indeed not that great, but the series started in the mid to late 90's, so give it a break. The artwork now is very great, and does a great job at showcasing movement, in my opinion.
The actions scenes are very nice, and the progression of characters in terms of strength and relevence to the story is very very nice. I hope that there are a lot of other people out there that feel like One Piece deserves more credit. :3