Gotta love how this thread, originally started out on the topic of whether to watch One Piece or not, to the differences between the 'Big 3'.
In relation to that, I personally prefer One Piece over the other 2. Naruto is good, but it's only those few times it has the "omg, that was epic." moments, and has a lot of fillers which really make me feel like dropping the show every time. I have nothing to say about bleach, it's just... No. One Piece, on the other hand, has a really vast .. world. It has real and imaginary aspects to it and you really can't guess what's going to happen next, what type of places they will visit. Their individual arcs all eventually connect up to a bigger picture (not being the pirate king, just some big arcs below that), and there are many times I have gone 'OMG THIS WAS SLIGHTLY SHOWN IN CHAPTER XX' when reading 100s of chapters ahead. Yes, the starting isn't that great. Animation is from the late 90s and the simple dialogue which make it seem like a mere gag manga/anime, really turns people off from watching it. So it's obviously the people who stick around to watch at least up till ep 20~30+ where they start to take an interest in One Piece. However, an arc that could represent not even an inch in the world of One Piece prob starts 80 ep into the series. And that's when most people (or so I have heard) start liking the series for its comedy and action.
None of the characters (main ones) have a useless role like you see in other shounen anime/manga. They might not all have equal amounts of power but they do have (or gain through the growth of their characters) interesting powers which they actually use, and not just stand around holding their weapon. Oh, and they actual go through events or training times where they 'power up' or enhance their skills, unlike other typical shounen mains who gain more strength through willpower.
And what I prob love most about One Piece is just how when you figure one thing out, there comes another 100 questions. The flow between each arc is so smooth that they do not even seem like separate arcs to begin with. Oh, and lastly about how One Piece continues to grow and improve. If you compare the first chapter with the latest chapter, you'd be in awe when you see the difference in art and detail, and the plot. One Piece may start out silly (well that's one of the main elements) but as you continue watching, a lot of elements and details change for the better.
All that said, I'm being biased because I'm a total One Piece fan freak c: If you've already read/watched up to the Skypiea arc and is still not the least bit satisfied or interested, then I suggest dropping it (sadly). The arcs and plot get much much MUCH better after the Skypiea arc, but without any interest after reaching that far, I doubt you'd enjoy it as much as many other fans do.
Last comment, FT is not One Piece. FT is really generic with a few of its own original aspects but its plot nor world are on par with One Piece's.