I dont have any servant that i like least.
It is not that i like all of them.
It is just that instead of wasting my time to think about whom i should dislike, and spewing unnecessary hate speeches whenever one is mentioned in a convo,
i rather spend the time licking on those whom i like. From my pov it is about showing basic RESPECT to the characters themselves and those who loved them.
Bad VAs? It has nothing to do with the character. Bad design? Artist problem. Bad stories? Blame the writer. Bad personality? They must at least have a soft side for certain someone. Being rude to MC/Chaldea? Try rethink their actions from the character's standpoint.
Whether it's hating or least favorite, the more proper term that i would use on a servant that isnt my cup of tea, would be No feel.