Showa era is mostly episodic with very little to no development. You watched one and you've watched 'em all with exception of the beginning of Amazons and Black.
You can watch em in any order since there's no proper continuity (with a slight exception with Kuuga and Agito, and the crossover shows (Decade and Zi-O) need you to watch the previous ones to (not) "enjoy" them)
From my experience and observing other people, I assure you that starting with post-Decade shows is a nocive road that could probably carry with you during the rest of the franchise.
For me, I suggest you to start with Kuuga then release order (at first, consider the movies and extras once you finish their respective show).
Why release order starting from Kuuga?
>It's basically a reboot of the franchise
>From that point, the next shows until half of Hibiki have noteworthy features and are unreplicable with todays production methods, what you see between 2000 to 2005 will never ever come back
>All shows from 2000 to 2005 (probably excluding Blade) are enjoyable by most people even complete outsiders, not because nowadays common denominators, but because they really have something to tell and the characters doesn't feel like humans with cartoon-behavior
>On a personal note: These mentioned shows were really effective in an attempt of detoxing of anime
>Past 2005 and specially post-Gaim (2013) you'll find out that these shows got stale with their stories and gimmicks, sometimes with absolute trainwrecks
>And because I see release order as a proper way to enjoy not only these shows but their evolution (protagonists behavior, antagonists, backstory, character development, morale, messages behind and etc)
If you don't want to get into the struggle of swallowing the entire thing? 2000 to 2005 shows is the peak of this franchise, and take anyone considering the post-Hibiki or post-Decade as better with a grain of salt.
Do you want the most essential stuff? Kuuga, Faiz and the first half of Hibiki.
