My favorite's always been Even a Monkey Can Draw Manga, mostly because of the blunt truths it presents.
"Your ideas will NEVER be original, it has been done somewhere else, and all series plagiarize eachother."
Anyway though, I had an idea for a manga once: (read only if you want to rage and facepalm)
it was inspired by League of Extraordinary Gentlemen, and involved a group of washed up heroes joining together to save the universe, the cast went:
Ichiro Sato: In his middle school and high school years he was a legendary martial artist who defended Japan from an invasion of alien martial artists. Now, at the age of 38, he's a washed up salaryman who spends most of his time in pubs and his filthy apartment watching fan service anime.
Sakura Mahoyama: In her younger years they called her Magical Arrow Sakura Tenshi, she fought with her team of fellow magical girls to protect the world from evil magical forces from another dimension. But with the passing of years most of her old team has died in action and she herself is a 31 year old, divorced, office lady without custody of her daughter who views her as a loser.
7"Nanako" Gamma: A nuclear limited production robot from the post World War II era designed to fight off attacks from Japan's enemies while circumventing the army disbandment laws, she's now a broken down relic of a by-gone era who has been forced to use parts from her dead sisters and scraps she can find on the streets to retain minimal operational capabilities.
Saito Sakurazaki: A veteran of the last war between earth an it's space colonies, though his side was defeated his spirit is far from it. Though his side was defeated, he never was and continues to command the Yamato and its crew as well as Arc Saber, his infamous mecha that fought the Earth Federation ace pilot to a stand-still.