First things first…
NINETY-ONE!?!?
C'mon Japan, sort it out!
So, and I never thought I'd say this, the under-rated Mazinger Z. One of Go Nagai's many revolutions in manga and anime. While Tetsujin No.28/Gigantor came before it, Mazinger Z laid the ground work for almost every giant robot show that came after it. In particular – the concept of a giant robot that is piloted. But I hope you know all that already. If not I point you to the internet as there's no point going into it when other people have already gone into such detail.
And now, the second best Giant Robot theme in the history of mankind: