One Piece is an outstanding manga by perhaps the greatest mangaka in Japan right now, Eiichiro Oda.
When I first checked this thing out, the story seemed immature and poorly done, not to mention far too goofy. However I was soon corrected; it's none of the above. Luffy wants to be pirate king, but his main goal isn't that at all: It's to have as many adventures as he possibly can.
This is interesting, because unlike Naruto he does not wish to be the leader to get respect or anything silly like that. Luffy doesn't care about respect or appearances, which makes him a rather unique hero, even among shonen. Anyway, there are a series of shorter stories in the beginning as Luffy gathers friends (read: nakama). A swordsman, a navigator, a sharp-shooter/liar, and a chef join his crew in that order.
Although others criticize him, he takes off for the grand-line with that 5-man crew of trusty but perhaps inadequate men (and woman). The grand-line is where most of the stories in One Piece actually take place.
After that, a series of longer stories begin, each one with fresh and varying characters. Most have a very touching background, especially new crewmates.
At first, the art may seem horrible to you, but that's only because you're used to things like Bleach and Naruto, which are much younger. As the series progresses (rather quickly) the art improves.
Not much to say here.
Honestly, if I could give an 11/10 here, I would have. This is Oda's specialty, which is saying a lot, given his unnaturally great talent. Not just the main characters, but even minor ones (Dalton, Red Leg Zeff, Bellemere, et cetera) are given heartwarming backstories, personalities, and emotions.
It's quite enjoyable, from laughs to tears, to desperately rooting for a fight. As it keeps things fresh and interesting, you will even find yourself guessing at who the newest crewmate will be, who the main villain is, and what fruit who has.
In short, this can only be referred to as "The Big Three" with Naruto and Bleach because of length, and by no means quality.