Most powerful Mecha to ever Exist
" Super Tengen Toppa Gurren Lagann can literally just step on the galaxy, so for raw power that wins out I suppose. Thing is that you have to think of the pilot as part of the mech's "power" and most robot jockeys are just too PC or weak-willed to truly use all the tools at their disposal, so I'm going to nominate Zeorymer of the Heavens as most powerful since its pilot, Masato Akitsu, becomes more or less a sociopath behind the controls and at some point pretty much just stops caring how many people his "Hades Attack" is vaporizing on the ground. "idiot sociopath
Not on a ridiculous size level, Master Pokey from Mother 3.
Lord Genomes Lazengunn was powerful, however after a thousand years of idleness he and Lazengunns skill decresed.Then we have Unit 01 at the end of EVA 2.22, which is something considering EVAs are usually powerful.
Strongest I can think of would be Demonbane, for obvious reasons. If I had to think of a runner-up, I'd put Unicron and Primus over STTGL.
Two abstract beings/ mecha representing darkness/ chaos and light/ order within the multiverse > Spiral Energy fused mecha piloted by humans.
Well I'm not sure if he counts as a mecha, but Ultron killed all the Avengers and took over several galaxies (I'm not sure what the biggest thing he's capable of destroying is though).
Then there's GaaoGaiGar and Getter Emperor. I don't know much about them, apart from that they're really big (Getter Emperor is the size of a galaxy I hear).