What do you think is that scariest? I'd have to say the Giant Ant, then Radroach, then Radscorpion. As you can see, I had a hard time at the beginning of Skyrim.
The Wanamingos.
They're so eerily enigmatic. We don't know what they were "made" from, where (canonically) they came from, or many other details, and they're rather formidable in their own right.
The Wanamingos, not exactly scary, but more like annoying, their tendency to fake death and hit critical in groups made me skip Redding mines on my second and third run, sorry Redding.
I have Aracniphobia, which makes Halloween hell for me. But I never found the rad-scropions scary at all. What really made me shit my pants? Those fucking Cazadors, that's what!
Also in New Vegas. Everytime I saw a red dot moving from left to right erratically in the compass I started running the other direction. Fucking Cazadores.
Scariest critters are the invisible kids in the FO2 european version. The scary thing about them is that there's no official fix for them...
(not to my knowledge anyways)
For me it has to be the radscorpions. They were the second critters I came across the first time I played FO1.
You start with rats wich seem not too impressive, and the you see a hell as s*ht huge scorpion coming at you with that giant sting. They weren't as hard, but I was freaked as hell of them even getting near my character. I used to shoot from afar and move away so they wouldn't get a chance to get near.