This has been and forever will be my favorite anime of all time. I am a bigger fan of the manga (I think because it comes straight from the writer Oda and really gets across his unique style) but the anime has done really well and I love to see the animation improving as the series continues. One Piece is a brilliant combination of unique characters, action, drama, humor, angst, and fantastic art, all mixed into one series written by a master storyteller.
I am an manga geek to say the least, reading through hundreds of series of manga and watching through a few series' of anime. I find a lot of them to be impressive and i am blown away by the art in a majority of them, but none have consistently kept me fascinated and emotionally attached to the characters and the world they are in as One Piece has.
Though the series starts off simple and lighthearted at the beginning (which might make it difficult to get into) it develops into a brilliantly complex plot. However, Oda somehow manages to keep the immediate story simple and exciting, while it is clear that he knew that events happening in episode 100 would be of major importance in episode 400.
Simply put, One Piece isn't simply a kids show with a bunch of crazy looking characters and ridiculous action sequences. It has a depth to it that I have found is lacking in a majority of the anime I've watched.