Subnatica must be played first. Below Zero's story will make no sense without context, and in my opinion, the first game is better all around.

Really depends on what you're into?

My go too single player recommendation lately has been Subnautica.

It's cheap, easy to learn, unique, open world, encourages exploration, base/vehicle building, and has a great story wrapped in mystery that you discover at your own pace. It's genuinely one the best single player experiences I've ever had. I highly recommend avoiding any spoilers before beginning your journey.