GIANT ROBO "The Day the Earth Stood Still", Yasuhiro Imagawa's celebration of the works of Mitsuteru Yokoyama is a fantastic OAV series, IMO. I watched them on VHS when they originally came out; the wait for that last OAV was excruciating...
I then watched the DVDs a few years later. Have the Blurays, but not watched them yet. 90S animation + classic 60s character designs + good story, imo. There are some great moments that I still think about from time to time even though it's been a few years since my last watch.
There's an unfortunate issue that the production company ended the OAVs early, so the end sets up the antagonist for the next story arc that didn't happen, but you get a satisfying end to this story arc, I think even if the setup for the next arc is left hanging.
I've not seen all the Ginrei specials (made between OAVs by the team using the Robo cast), though. Have them, just never sat and watched them yet (wanted to do it with my next rewatch of the OAVs).