Personally I’ve got a laundry list of servants who fall under this list, but i’ll list the first one that pops into my head; Penthesilea. I got her last year during her first banner and thought she was okay for the most part. Didn’t like that she had no defensive options whatsoever, but I worked with it. I had a little more investment in the Death Race event where she helped with some bonuses she had, but otherwise didn’t think much else. That changed with the Himeji event where she had one of the strong bonuses, and I was able to try her out with a lot of fun strategies. She also had a fun place in the Himeji storyline (this was also the point when I was finishing up with the Nightless City campaign) so I was quickly being won over. I was completely sold when she helped me completely decimate one of the challenge quests, and since then she has been one of my go-to berserkers for questlines (alongside Heracles).
Best hothead for me.