Place 1: Tyranids
The greatest danger to the galaxy currently comes from the Tyranids. They have previously destroyed galaxies and quite literally devoured them. According to rumors, our Milky Way is even the last galaxy they have not yet destroyed.
The voracious bug aliens only seek to consume everything valuable in the universe and thus strengthen their swarm fleet. Their greatest strength lies in their ability to quickly adapt to adversities and literally drown the enemy in their own corpses.
When a Tyranid falls, the swarm consciousness does not care. The corpse is eaten afterward, and new Tyranids are produced. There are very few records of battles in which the Tyranids have actually been defeated.
Even the Space Marines rarely plan to combat these Xenos. When Tyranids infest a planet, the Imperium merely attempts to save as many citizens as possible. After that, the system is abandoned.
Additionally, they have the ability to infect humans and thus establish “Genestealer” cults. These act as sleeper cells on planets of the Imperium and assist with the invasion.
In the community, there is a general consensus that sooner or later everything will be destroyed by the Tyranids. And that is probably true unless multiple other races band together against them. In direct comparison, they are likely the most powerful faction: Tyranids are one of the biggest threats in Warhammer 40,000, and there’s a reason for that
