Exactly what it says on the tin; let’s discuss the servants that you considered to be weak (or at the very least didn’t think could fulfill a certain role), yet they surprised you when you took them out for a spin.
For me, it was Rider Martha. She spooked me enough times to get her to NP5, but I barely ever used her, because I considered her a switch-in servant for CQs for her team cleanse skill.
Then, due to the current downtime, I decided to take her for some door knocking in the dailies, if only so that I could see her animations, and I was completely floored when her NP dealt more damage (with the same buffs) than Ridesthar’s. Then I tried to see her ceiling, so I threw Merlin into the next run, and her NP dealt 170k+ damage per enemy! Sure, this was an upgraded NP, at NP5, but considering how low her attack is (and she wasn’t even completely Fou’d), I never expected that kind of damage output from a gimmicky CQ support servant.
What is your story? When did you have a similar moment, where you had to do a double take and reconsider your opinion about a servant?