Mecha anime is an important part of the animated genres. It incorporates the best of high fantasy and science fiction themes, but is beloved due to the fascination fans has for robotics today. There are many well written mecha anime works, so choosing the best is done at each fans discretion. Although if forced to choose, the following would be great ones to take note of historically.
Knights of Sidonia is an important part of the mecha anime mythos. It speaks as a series to the quest for humanity in the alien unknown, through the help of a robotic protective force. In the story, robots called Gardes kept Sidonia from falling into damnation. This war for humanity is fought by mecha for many generations, so the story contains historical intrigue and great landscapes of science fictional diversity.
Code Geass is unique and a branch apart from much sci fi anime. It is the tale of a great conflict and rebellion in the warzones based on many mythological tales combined. A few people from the former Japan have access to mysterious power which comes from their royal lineages. This is the starting point of a world conflict that shapes the world, making this far from a mindless or typical mecha anime. Although the mecha are actually a secondary part of the plot line in this great, but subtle exploration of the mecha genre.
Tengen Toppa Gurren Lagann is an epic for the ages, but stands out in the world of modern anime. It is well known as the most expansive anime ever made and truly rocking in the realm of mecha anime. The fate of humanity is held in the balance during times when the sky is gone, and the world is underground living in darkness. All is about to change because of an ancient technological relic that is discovered by a young digger, but no spoilers beyond that.