I summon Jeanne Alter to wreak death upon my enemies! Hush, Grand Order me. My general inclination is to go for Caster or Assassin. Caster because magic is awesome, and Assassin because then I can steal the Holy Grail and hide it or make a wish before anybody else can, and then nobody has to die! My villainy is unmatched in this world!
But, anyways. Assuming I'm fighting... I've got a few ideas. One that's been recommended to me due to probably having a similar wish would be Asriel, of Undertale. To be clear - not Flowey. Asriel. He'd be, in all likelihood, a Caster-class Servant, and assuming his Noble Phantasm didn't disqualify him, it would probably be to assume his God of Hyperdeath form. Not only is that going to be rather difficult to defeat, he'll get stronger each time he defeats someone. Considering how much a human soul does for a monster, you have to wonder what a Heroic Spirit's does. And then, at the end, everything gets fixed because bleep you war. ... This is assuming he could ever be persuaded to actually participate, which is unlikely and which I honestly wouldn't mind if the other Masters and Servants would leave me out of it.
But, let's see... beyond that... most classics are out. I don't know many, and I'm pretty sure a magic lance would reach out from my phone and stab me if I tried to say Dracula. But... hm. Trisanna Chandler, Caster-Class, from the Circle of Magic. Hope you enjoy getting lava in your face!
I suppose I could take the Mary-Anne route to justify summoning, say, Maka and Soul, Lancer-Class Servants, or Death the Kid with Liz and Patty, Archer-Class Servants. Or I could basically ask for my own death and summon the Berserker Asura. Which... I wouldn't do, because I don't particularly feel like passively being involved in the destruction of cities. Of them, I think I'd probably opt for Maka and Soul.
There's also Harry Dresden, Caster-Class Servant, but... let's just say a book I just read slightly sours me on that idea. But he might work.
As I've assembled far too many and need to go back through everything I've ever read or watched, I suppose I'll let it go for now. It feels like the perfect Servant is sitting just out of thought's reach, though.