I've been trying out Raiden, Kotal Kahn, Sub-Zero and Kung Jin.
I need to play more with Kung Jin, but I'm liking his Shaolin variation.
I'm not too wild about Kotal's "Sun God" variations (at least in story mode), and had to get used to not having his wooden bat / saw weapon in that variation.
I need to try out his War God and Blood God variations, though.
Sub-Zero is fast as hell, which I absolutely LOVE.
I couldn't stand him being a slow, zoning, power character as my playstyle is more rushdown, and feel like I can use him better in this game (at least with the Cryomancer and Grandmaster variations that I've tried out).
I have yet to use the "Unbreakable" variation, but I will try all 3 of his.
Raiden is VASTLY improved for me, as well in MKX, and I love how fast he feels.
He also has better combo strings in MKX, IMO, and I LOVE his Displacer and Storm Lord variations.