Aright, I gotta be honest here. I haven't watched a lot of anime. Not nearly the amount most people on here watch. Hell I just started watching Attack on Titan because my sister made me.
I don't like One Piece. And no i'm not talking the 4kids dub i'm talking the japanese sub version. In general, One Piece. Manga, anime, all of it. I mean it has a decent story, and from what I can tell the characters are pretty strong, and it's got a huge following...I just couldn't get into it. I really didn't like the art style and the story was a bit to...wacky. I'm not saying it was bad by any means, it's just the one I've watched that i've gotten the least enjoyment from. That and DBZ but I wouldn't dare bring that up here.
Like I said I haven't watched a lot of anime, so I probably haven't seen a lot of the one's that are going to be topping peoples lists. [I haven't seen any of the ones already mentioned in this thread.]