I’m in the middle of rewatching Giant Robo (which I watched for the first time a LONG time ago… it’s a pretty old OVA series.) It does stand up well on the rewatch, even after I’ve watched a ton more anime after seeing it. In some ways, you could say it’s kind of a mash-up and kind of nonsensical, but in a totally over the top comic book superhero way? Yes, actually, that makes sense, because it’s kind of a mix of science-fiction technology (especially the titular, creatively-named Giant Robo), people with magical or psionic powers, organizations which openly state their goal as “taking over the world,” etc. There’s a ton of fighting robot action, superpowered battles, and unabashed drama: it’s pretty plot driven and filler-free, with all the battles moving the story forward. The wikipedia page is actually pretty good and doesn’t have too many spoilers (I advise you not to spoil yourself, BTW).
I have only read one of Yokoyama Mitsuteru’s manga in full (Xiang Yu and Liu Bang) but I definitely would like to get around to reading more sometime. I’d also like to try and read the original Giant Robo manga and the Giant Robo: the day the earth burned manga.