Now Mecha is an interesting genre. Not only because the concept of fighting humanoid robots is cool, but also because that's never ALL that the show is. Mecha can be so much more. From really sad, to intellectually stimulating, to incredibly ridiculous. My favourite mecha anime definitely falls into the last category. I think you all know what it is.
TENGEN TOPPA GURREN LAGANN
In my opinion, this is by far the best of the mecha genre. It is completely ridiculous, in scope, characters, everything. The main character, Simon, is but a humble digger until he one day finds a face (yes, a face) buried deep underground. He and Kamina, the manliest character ever created by anyone, ever, use it to escape their underground home. Eventually, they realize it is because of spiral power, that they are able to do so.
In fact, that is one of the coolest parts of this show. The entire show focuses around the spiral. Weapons are drills. DNA, with its spiral form, will one day destroy the universe. Even the story has a spiral scheme to it, starting of small and accelerating outwards rapidly, until eventually, their mech's are the size of galaxies. Every movement defies the laws of physics, and it completely defies logic. But who cares? "Kick reason to the curb and do the impossible!"
I can't get over how awesome and ridiculous this show is. Watch it, if you haven't already. To top it off, it has world-class art and music.