Never realized Luffy was so incredibly overrated until I found fanboys in comic forums saying he could beat Superman, Goku, The Flash etc. I mean, I prefer One Piece power levels over Marvel and DC (and Dragon Ball) any time, but if there are so many fans out there who think he would even be left standing for one second against Superman, he must be pretty overrated. Still, that is purely in terms of strength. Character-wise, he deserves all the fanboys he gets. Such a great leading man, all his stupidity and his faults be damned.
So I'll nominate Ace for now.
I put Ace basically because everyone seemed so damn shook up about him dying, while I just felt like a heartless prick who was simply mildly surprised that someone died in One Piece, non-flashback. And everyone still hail him as if he was the greatest thing to ever happen to the manga. I thought he was cool, yeah. Not really the most well developed character even with all the flashbacks. I was pleased enough everytime I saw him, but never understood what people saw that I didn't that made him so great. I guess I'll never quite get it.
(But then again, I nominated Shanks for Favorite Male Character and I'm well aware that besides chapter 1 he hasn't exactly proven himself more than Ace. Still, he's not the one who's being raised to saint-dom by the entire fanbase. Ah, well.. I digress.)