For me, that would be Jason.

I do have Muramasa, but I just love using Jason if not only because of his Cost lol

It is also satisfying farming with 1* servant.

Jason and Izo for me. Jason had some great character moments, but Izo is just a chad.

Bro has SSR level drip on a 3* rarity.

None. At least there is no character that I started liking only after their story, as I like mostly for design only. I don't really use any of them tho, except maybe Katsu, who I use sometimes with Nobu

I like Izou because of gameplay. Special damage against Humanoid + upgraded NP + special damage against enemy with human attribute made him really strong. Not to mention he also have massive crit damage up.

Salieri, really like the animations and sound effects. There are a few others I'll use, but they are useful, not loved.

Leonidas. I even have him grailed and max bond.

I probably like Xu Fu the most in terms of character, but overall none are among my favorites enough to grail any. For gameplay I use Arash, Chen Gong, Cu Caster a lot, and Xu Fu occasionally if she's satisfactory for a farming comp. Other than that I pretty much only use them to save party cost, or as a multicore option for an unorthodox farming setup.

Eric Bloodaxe.

Hans