Been thinking about the plagues and which one, besides the last one, would be the "worst"? I would have to go with the flies, especially if they were the "biting" kind. I hate those things.
Then again, having frogs everywhere wouldn't thrill me either, especially since there is apparently a smell factor.
Being pelted by hail would be no picnic either.
The darkness thing would be scary but not physically harmful. Except there must be some physical meaning of a darkness one can feel. One of the LSB footnotes questioned whether this meant it was a sandstorm, but I would think the text would have said so if it was, since all the other plagues were quite clear about what they were.
Let's face it. No one would want to experience any of these things, and we'll just leave it at that.