Missed the chance to comment on Manta, but he is a great villain.
As for Red hood, I do think making another character from that identity was a good idea. As for Jason Todd, I have no use for him except for the Pre-Crisis version who was just a Dick Grayson clone but was at least inoffensive. But his death-and that the fact that people voted to see a teenage boy brutally beaten to death with a crowbar-was disgusting and was the point when I understood that comics fandom had become largely infused with sick deviants.
Never liked that they retconned Jason Todd since he was heavily connected to the Killer Croc and Nocturna storyline, but I must admit that Max Allan Collin's idea of turning him into a lonely street punk (of course just an homage/rip off to Junior Tracy) actually made some sense, as Batman explained it in various comics.
Although Jason Todd is one of the characters where the common perception is usually wrong because most people haven't read his original stories, they have just read about him what other people said.
For example, the whole idea that post-crisis Jason Todd was obnoxious, unstable and violent and that his whole path to his death was a continuous development is just wrong. The Jason Todd written by Collins or Barr was definitely not like that, in some ways (especially Barr's version) he was even more playful than Dick Grayson ever was.
This didn't really start until Jim Starlin took over whose complete goal seemed to be to make the readers dislike so he could kill him off, with stuff like killing the diplomat's son. They even drew the flashback of Jason confronting his father's killer, Two-Face totally different from the original story so he would look more aggressive. And then after Jason was dead Jim Starlin let Batman describe him as "the best" and stuff like that, except that we never saw something in his stories that justify this. Apparently he liked Jason but hated the idea of Robin.
Funnily enough, rumor has it that Warner was pissed off that DC killed Robin and then Denny O'Neil & Co. made Starlin the scapegoat and fired him.
Never really cared much about the Red Hood persona.