Beeswax is one of the oldest leather conditioners (and it is used in many other products including wood polish, candles and cosmetics like lip balm).
PROS:
In its natural form it's free of chemical solvents
Protects against moisture
Glows when polished /brushed
CONS:
It's expensive
It may darken leather (this could be a good thing, depending on your goals)
I haven't tried this yet, but here's a recipe for making your own natural leather conditioner using beeswax, coconut oil, almond oil, and castor oil:
https://www.instructables.com/id/How-To-Make-High-Grade-Natural-Beeswax-Leather-Pol/