I like Sub-zero, where he asks if that's his brother

Kitana/Kitana where the first Kitana ends up sounding exactly like Mileena.

Reptile versus Reptile:
R1 - Chameleon...
R2 - It is I, Shang Tsung!
R1 - Show yourself, coward!

Erron versus Erron:
E1 - I'm flattered.
E2 - I'm you, idiot.
E1 - I'm no longer amused.

Quan Chi versus Quan Chi:
Q1 - We mustn't fight.
Q2 - I'm afraid we must.
Q1 - You were my only friend...

Quan Chi's is hilarious, and I like to pretend that with the first one, whoever's playing as the impostor is technically, for that fight, playing as Shang Tsung. Another one that I really like is Raiden's, where the alternate Raiden introduces himself as "Raiden, Protector of Chaosrealm!", to which our Raiden responds "The flow of time has indeed changed." Something about him changing allegiances like that is interesting, and expands on the multiverse theory.

I agree. Shinnok and Scorpion's also give hint to a multiverse possibility.

Shinnok's references one of them being the Shinnok clone sent into the melee during Armageddon.

So Kitana fights another Mileena in other words? I also didn't know Kano had clones.

Something about the delivery in Ferra/Torr's mirror matches are funny/cute.

1: What's big idea?
2: You rider, like me?
1: We fighters, Go Torr!

1: Why you look like we?
2: Why YOU look like WE?
1: That's no funny!

I find all of their intros hilarious. Delivery is on point