Have you ever loaded into a game and seems a HB that you go "ah sh*t, not this guy"?
I'd have to say, I think it'd be Gus the Bus. Quadfather doesn't scare me anymore because I can TFL him fairly without too much difficulty (on inside runs, not outside ones). King Henry worries me a bit, but I haven't had too much issue with him recently. Flash can also be scary, but Flash's X-Factor requires a move to be made first, so you could still get a tackle if they don't make a move
Gus scares me the most A) because of his xfactor and B) he gets Backfield Master and Bruiser for free, making him a threat in the passing game