"Mars" by Mitsuteru Yokoyama is a short, underappreciated classic.
I will avoid spoilers, but the story starts off slow but gradually gets faster in pace until it gets out of hand before the abrupt, but satisfying, end. Mars' dilemma and the morals all perfectly tie together, and with the ending make it an overall sad, but fun read.
The art is simple, but extremely expressive. Robot design is exactly what to expect from Mitsuteru Yokoyama, rounded and simple, and very shiny. The character design is fun, Mars with his flowing hair, the middle aged reporter, and all the crusty old men villains are pretty good.
I think this manga is a very enjoyable read, and a pretty quick one at that. Check it out whenever you can, you'll have a fun time. Now, if only it could have a complete, accurate anime adaptation, eh?