I only ever played MK 3, but damn did I play the shit out of that game. I was always Baraka. He was just so messed up. 

I've always been big on Liu Kang. His bicycle kicks are just great. Plus I see him as faster than some of the other fighters

I only have favorites not just favorite. Id say liu kang, raiden, and scorpion.

I play as Liu Kang because of the ridiculous noises that he makes. Maybe not the best criteria, but it works for me.  

Scorpion.

Liu Kang in MK1 and Kung Lao in MK2. Never had a favorite in MK3.

reptile, manbeast who spits corrosive acid, camouflages, fires forceballs. And is simply trying to save his species.

Kung Lao was my go-to guy during my time with MK. Maybe it's because I'm a Bond fan, but there's something so damn cool about a razor brimmed hat. 
 
Sub-Zero was next because he reminded me of the Predator by ripping off people's heads. This is how I value MK characters-by how much they remind me of things I already like.

I was a Kano fan. I'd just spam the knife throw. I wasn't real great.

It used to be scorpion mainly just cause I like the color yellow :P. But that was when I was young, now its either smoke or cyrax.

I always liked the Cyborgs. 
 
Especially Smoke, he was the only one I ever beat MK3 with.

Smoke from MK2

Always liked playing as Sub-zero

Sub-Zero and Scorpion 

Reptile because he reminded me of Piccolo.

i love me some reptile

Even though it was the house rules that you had to go Random Select, I was a Kung Lao man... especially because he had what my mates and I dubbed the "Side of Beef" fatality:
  

These are my favorites from each game, and this isn't related to their movesets or who I like playing most with, but just their characters overall.
 
MK1 - Reptile (The ultimate secret character. Legendary)
MK2 - Baraka (Just so freaking cool)
MK3 - Kabal (Let's face it, most of the MK3 characters sucked.)
MK4 - Quan Chi (Sorry, Jeff)

Always Be Sub-Zero

kano

Scorpion and Kabal.

Sub-Zero because he had the easiest spell. 

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Don't think you needed to state 'from the originals' as there were only one or two good new characters post-MK3. 
But it was always Sub-Zero.

I've played every MK up to MK4 where I stopped due to not liking the new systems, characters and overall direction the series had taken. My absolute favourite of the classic games is Mortal Kombat II and I mastered Kitana, Baraka and Kung Lao.

Kung Lao