Yeah, I've had that feeling that One Piece isn't so popular around these forums, but that could just be me. Anyway...
I suppose I might as well join.
How should I start...I'm not exactly sure how I became a fan of the series, considering it was back in 2003, I think, when the series started airing in my country. I've watched it regularly until Water 7, which was when I had no time for it anymore. Since then I haven't really bothered watching the anime, but I'm still reading every chapter when it comes out. For a mainstream series it's preserved its quality really good and it got a lot better, too (although I'm not a fan of the fanservice; at least it's not as annoying as in Fary Tail xD).
You know what I really like about the series? It's the way it's designed. Instead of this really good guy, who tries to save the world from all the bad guys, in One Piece it's pretty much the other way around. While the pirates are presented as the great heroic explorers that are chasing their dreams, become famous, find One Piece, the World Government (and the Marines) are presented as evil and corrupt. I mean you just need to look at the beginning of the series, where Luffy meets Morgan, who likes to terrorize the island where his base is located, even though he's a marine. Sure, even a lot of pirates are evil, but keep in mind that pirates can do whatever they want, while marines can not.
If you'd asked me about my favorite character a couple of years back I'd totally chosen Zorro, because three swords; now it's not as easy for me to chose, because they all have something that's kinda likable (although some of their designs are a little bit strange, especially after the time skip)
But if I had to go with one character...probably Sanji
He has an interesting fighting style and an hilarious weakness; also he's wearing a suit. And I still hope, Mr. Prince makes a comeback xD