While there are many settled worlds that are classified as dangerous to standard humanoid life, there is a world that exceeds them all and is the only garden world in the known galaxy classified as a Type X planet: Green Hell.
While it is a first glance a lush green world with a standard NiOx atmosphere, liquid water and an abundance of native life forms that would warrant the Garden world classification A 1 it is those life forms that make it different.
There are worlds with tremendous storms, those that are tectonic very active. Worlds with dangerous life forms and other treacherous conditions; yet none are comparable.
Green Hell is covered to sixty percent with deep jungles and every life form, from the smallest bacteria to gigantic floating living clouds everything has an arsenal of natural weapons and all are extremely lethal. No known species, be it Pertharians, Nul or the massive Thala is able to survive unprotected for more than a few moments on this world. The strongest corrosive acid known is produced by local ants, the deadliest poisons; bio toxic compounds far lethally beyond even the poisons used and identified by the Shail are produced by animals and plants of that world.
Yet Green Hell has a small Union colony, mostly to support an Exobiology research outpost of the Union science council.