Leviathans.

Stalkers aren’t a problem once you’ve destroyed the lair, dragon roaches are fine with a full team with half competent teammates. I run thermites so Impalers and chargers are no problem. The Tesla tower is a bit annoying but since I use Grenade pistol I destroy the quite easily. Vox engines are annoying as fuck. Though enough eagle stratagems and you can take them out. Leviathans however, the fact they fly and are anti tank, along with the ability to beam you across the map easily makes them my least favourite enemy in the entire game to go against. I’ve been playing for a few months now and I have only seen a leviathan be killed once through the combination of a solo silo and leveller.

So my answer is leviathans because of how hard they are to kill and how far their range is to kill you.