Gotta go with Cherno Alpha.
I just love the premise. I can see a Russian scientist explaining it: "Da, we know Amerikanskis put missiles, swords, fancy cutting-edge speed in giant robot, try to outsmart Kaiju. (shrugs, sniffs) We have yet to meet Kaiju that can outsmart robot running straight at it and beating it to death with bare, how you say, fatherfucking hands."
It is perhaps the best extension of the movie's tagline, "go big or go extinct." No gimmicks, just heavy steel on top of more heavy steel. No wonder it survived so long.
It gets bonus points for a unique design. Usually mecha have small heads to make them look more massive. Somehow the oversized head makes Cherno seem even larger and more intimidating, rather than goofy and misproportioned.
Second place is Crimson Typhoon. A third arm is such a simple gimmick but it's exactly the kind of distinct special move you're looking for in a kaiju movie. Add spinning sawblades and you've got pure gold.