From everything I've seen from in-state reporting across multiple Senate campaigns, Sara Gideon seems to be the most obvious choice. Absolutely botched.
Bulloch was a state-level D running in what ultimately became a R-friendly environment in a very federal R-leaning state. Saying he was the worst because he lost a federal race by 10 points in a state where federal Democrats routinely lose by 15 points is shortsighted. Gideon underperformed Biden by nearly 20 points against a shaky old woman in the minority party.
I wouldn't call Collins a "shaky old woman". She was a four-term incumbent in a state that likes to split tickets who also had arguably the strongest personal brand of any sitting Senator. She was never going to be a pushover like Gardner.