These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Demonic Spiders: The evil spacemen, no doubt. This is because A) They have something approaching a survival instinct when it comes to dodging your shots, but more importantly B) They're the only enemies in the game that regenerate after being killed
Not only that, but their projectiles fly really fast, making it nigh impossible to dodge if one is flying head-on at you. They also do a ton of damage, and even if you can stay out of their line-of-fire, they'll still try to fly in and collide with you.
The hopping robots that constantly spew bullets at you. Depending on where they appear relative to you, they're either not that much of a problem, or an utter nightmare rivaling the evil spacemen.
The floor and ceiling-mounted robots that shoot lightning when the player gets near them. The player can only get past in short intervals without destroying them, and if they get knocked into the lightning they can very easily get locked into a severe Cycle of Hurting.
Goddamned Bats: Most of the enemies in the Fortresses, to be frank. Most of them aren't that dangerous individually, but their tendency to hang out in groups, combined with your very short damage invincibility, makes it easy for you to take gobs of damage with little way to avoid it.
The indestructable turrets that fire homing shots definitely take the cake. No matter how dangerous everything else is, at least you can actually kill them.
Good Bad Bugs: Enemies in the Air Fortress, especially turrets, occasionally "spawn" pre-destroyed. Given that, with a few exceptions, destroyed enemies in the game do not regenerate, this is pretty helpful.