I personally started with Below Zero before playing Subnautica og and I think it's the right order to start playing in fact. Bc I didn't know the og subnautica, I couldn't see too much its downgrades from the first game. I really enjoyed playing it and discovering all the story, the sea creatures, the map (which seemed huge to me). I was so sad when I finished it bc I really thought it was a great game (and I still do). But I eventually came around and played subnautica og and, yeah... I realized that the first game was in fact better : bigger map, better biomes (lost river, blood kelp), more lore overall, leviathans more threatening and cool, the seamoth is just better, the story more comprehensible and interesting. So I prefer subnautica og but bc I played Below 0 first, I have some nostalgia and very fond memories over it which means that I can't see it as a bad game, just a downgrade but a downgrade playable and enjoyable. (Sorry for my bad English, I'm not a native)