Giant Robo – The Day Earth Stood Still is a OVA from the original Giant Robo manga1. The plot is set in a alternate future where all the energy is supplied by a miraculous Shizuma Driva that replaced Fossil fuels when renewable sources weren’t successful. Apart from that interesting idea (and the problems that it brought), GR is a mecha anime and the main characters have super powers.
Giant Robo has been called “one of the true timeless classics of Anime”, an epic in the vein of Homer’s Iliad and Wagner’s Der Ring des Nibelungen. Mike Crandol of Anime News Network says Imagawa “takes the best of the old and mixes it with the best of the new to create the definitive giant robot story.” John Huxley of Anime Boredom concludes the series is “the super robot show as it was in your mind’s eye, a perfect combination of the old without the disappointment of reality.”
I wouldn’t go that far, but it is indeed an epic anime.
1 Doesn’t have anything to do with homonymous movie