Can I do multiple choice?!
I was very perplexed,
then I voted for most favorite--
Rider.
Spoiler for comment for other characters:
Caster: She is CUTEST! ...But, she lacks a bit monster, imo.
Rin: Tsun-dere! Twintails!
Berserker: Seriously, I'd thought to vote for him.
Assassin(Sasaki Kojiro): Kool! He's my FACTOR. w
Gilgamesh: I guess, the potency of ALL-STORY exists before Babylonia.
Why could you think you're the FIRST, Gil?