Mazinger Z is definitely one of the greatest songs ever, when I heard Aniki sing it live, I could feel the mecha spirit in his powerful voice. Also Minmay's Ai Oboeteimasuka, the song that made me cry watching an anime for the first time, its one of the most beautiful songs ever.
For us mecha fans, there's one song we all love:
Yuusha Oh Gaogaigar OP
Masaaki Endoh - Yuusha Oh Tanjou!