I haven't seen Monster but plan on checking it out soon. Heard good things about it, but
you're seriously comparing Fate to a known classic
And I agree that in Stay Night the characters outside of the servants are forgettable, but I don't think that's the case in Zero. Kiritsugu and Kotomine aren't servants and are pretty complex when it comes to characters in anime imo.
But let's not group Zero and Stay Night together cuz Zero is clearly the better anime. It's gonna be ranked much higher on my list than Stay Night will be