One Piece #58 set a record for first-week sales by selling 1,852,541 copies, more than any other book since Oricon started posting sales charts in 2008. The last three One Piece volumes also rebounded back into the top 100!!!
FLCL has bad art? first of all if we're talking japan it recieves no fanfare, like at all. its like one of those really good series that are kind of unknown/forgotten in the main stream in japan but really popular in say the US because everyone watched adult swim for their anime during that time. cowboy bebop is similar.
Though imo FLCL has one of the best art direction I've ever seen. As for One Piece, while I think the anime is pretty good, all jump anime has its bad moments just for being so long and drawn out. However the manga really is great, though it took a while for OP to reach its popularity, it's because you have to get PAST a couple of things to appreciate it.