>>23327549
I can't decide between Gunbuster and Giant Robo The Day The Earth Stood Still. Zeorymer and Dangaioh are pretty great too and I have a soft spot for Dragon's Heaven because I love the Moebius inspiration and Kobayashidesigns.

Honorable mention to New Adventures of Byston Well because it has some of my favourite mechanical designs ever...but the animation is not good so I can't say it's a good OVA.

I am really bad at narrowing favourites down.