Honestly, Voyager sings to me. And you may be asking: Kim, there's only one servant in the Voyager class, and it hardly even counts, since Voyager is a weird unique guy in Fate/requiem, and it would fit the question better if you said you would summon a Foreigner class creature because that's what Voyager gets assigned in F/GO.

But I want to summon little floating baby..... I think he's so awesome... His dynamic in requiem with Erice as an extremely competent and combat-capable Master while he just a baby that has to be protected is really good, I like it a lot. So that's my pick. Second choice is Avenger and crossing my fingers for Salieri and getting Ishtar probably.