They aren't so different from each other.
Shirou can only find true joy from saving and helping others while Kirei only finds true joy from hurting others and causing pain.
They both work so well is because they are both natural enemies towards each other but at the same time they could have been the best mentor student dynamic.
For me it's Unlimited Blade Works with Lie Like Vortigern coming in at a close second place.
Reasons:
1.Emiya Theme.
2.UBW Aria Chant.
3.Allows for top tier Aura Farming.
4.Coolest Ability.
5.Its the ultimate underdog ability.
6.The Infinite Tactical Versatility.
7.Broken Phantasm Potential.
8.Visual Aesthetic.
9.The Perfect Counter to Gilgamesh.
10.It Tells a Story
How can Shirou's Degraded B rank Hrunting match Gilgamesh's orignal A rank Hrunting when they clash as projectiles?
He doesn't have any legend of his own.
He doesn't have a age of gods human body.
He isn't a demigod or has Mysthical lineage.
He doesn't have any true magic.
He doesn't have broken mystic eyes.
He doesn't even have a Noble Phantasm.
He has so low Stats.
Even while having so many disadvantages I still can't believe this guy is a Low A To Mid A Tier heroic spirit.