You would be hard pressed to find an anime fan that has not at least enjoyed one anime of the mecha genre. However finding good mecha manga is a little bigger of a task. The drawn mech does not embody the action quite as well as the animated mecha does. However, there are still plenty of decent mecha stories out there.
I have also provided recommendations for mecha animes as well, most of which have very decent mangas as well. However, for the sake of variety, I have left them off this particular list.
Pluto follows a Europool detective named Gesicht as he tries to uncover the mystery behind a string of similar robot and human deaths. The case only becomes more puzzling when the murders are are suspiciously pointing to being done by a robot. A chilling thought as a robot had not killed anyone in 8 long years.
This story is actually based off an arc in Osamu Tezuka's Astro Boy but from the legendary Naoki Urasawa who created such thrillers like Monster and 20th Century Boys. While you may think this to be an intelligent romp through robotics akin to Isaac Asimov, but the discuss of robotics is rather simplistic. It focuses less on the science and more on the story. Fans of noir mystery and science fiction alike will enjoy this tension filled narrative with a surprise and awesome cameo from Astro Boy himself.
During summer camp, 15 kids find a grotto by the sea. After exploring, they find computers and other electronic equipment that by all means shouldn't be there. With them, there is also a man called Kokopelli. Kokopelli is a programmer working on a new game, in which a giant robot has to defend Earth against 15 alien invasions. He persuades the game to sign a contract to test the game, all but one signs and the next moment they awaken on the beach thinking nothing had happened.
Bokurano is probably the most nihilistic thing ever written. In essence, it's a series of 15 short stories about these 15 kids who are trapped in a competition where they have topilot a 500m tall robot. If they lose, the world is screwed. Gradually, through a series of well written and shocking twists, everything about their mission ends up being bad for them and pretty much everyone around them. It's a brutally realistic depiction of what you'd get if you actually did put a bunch of middle schoolers in charge of this sort of thing, like Lord of Flies but with pictures. Some are wannabe heroes, others have mental breakdowns, at least one goes on a murdering rampage, and a few just don't care. It also turns out that people don't like it when you fight in the middle of a city and cause 10,000 casualties. Which is funny, because during the Transformers movies I pointed out the same thing about all those potential casualties. My friend, the Transformers fan, refuses to watch movies with me now.
Saikano (also known as Saishuu Heiki Kanajo)
Shuji and Chise are two Hokkaido high school students barely starting a relationship with each other. Then, with no warning, a squadron of bombers from an unknown country destroy the city of Sapporo. All hope is lost when suddenly, the bombers are destroyed by a small red light floating across the sky. At this point, Shuji finds out that the one responsible for eliminating the enemy forces is Chise, who reveals herself as a cyborg engineered for mass-destruction.
Get the tissues and prepare for feels. Saikano is punch in the crotch after punch in the crotch. First the couple is happy together, then they are torn apart, then they are nearly killed. There are so many tears in this manga that they flood out and leave a wet spot in your lap. Many people think the anime adaption goes a little overboard on the sheer emotion, but I believe the manga has a more favorable balance and in general feels more real. The artwork also takes a refreshing "sketchbook" sort of feel in which the lines are kept light to create a more angelic feel to the mechanical Chise and more realism for the average Shuji. Thought, the sketchiness is not so over the top that some things look unrecognizable like, for example, Blade of the Immortal.
Kurogane no Linebarrel
The story takes place in a fictional alternate future and centers on a junior high school student named Kouichi Hayase. Kouichi has found and escape from his life of being bullied by daydreaming about being a hero. However, after an accident, Kouichi finds himself bestowed with a girl and a giant mech called linebarrel. This leads the young many on a rollercoaster ride of epic battles and heroic adventure where he meets new friends and must deal with the repercussions of a hero's actions.
This story has been adapted into an anime, however where the anime has fan service and filler stories, the manga is much more serious. However, fans who watched the anime first will find that Hayase is much more of a likeable guy Hayase was from his anime counterpart. This is a beautiful manga with detailed mechs and battle, but takes a dark turn in the story that can please many fans of a serious story.
In futuristic Japan they created an armored suit call Infinite Stratos to be used for outer space exploration, however it was later adapted for use as a military weapon. However, this suit can only be piloted by a woman or so people thought. Along comes Orimura Chihuyu, the only male able to pilot the Infinite Stratos. So he is sent to the Infinite Stratos piloting academy where his instructor is none other than his own sister.
If you are not a fan of fan service, this may not be the manga for you. I tried my best to avoid it in the description above, but it is quite the harem manga, in which almost every girl on campus is vying for the love of Chihuyu. If you can look past the gratuitous fan service, there are a lot of awesome mecha clashes n this story. however, there is not a lot of substance.
These are just a few of the best selected from my personal manga library. If you have been fortunate to see all of these or have a specific sort of manga in mind, leave a comment. I can give more suggestions if you want them!