When I first started playing, Quan Chi was actually the easiest for me. His strings require you to figure out the basics like run buffering, overhead-low mixup, and juggling. Those were things I struggled with for so long but after picking him up and getting used to everything, I slowly got the hang of it (around a week or so).
From there, I went on to Kitana due to her forward advancing strings and their duration. One thing about MKX (and probably a lot of combo-heavy fighters) is you can cancel a move into a special depending on your timing and reactions. So, for example, if you do a string that's three hits, but the 3 hits take a little while to complete, it buys you time to figure out what you should do on hit confirm vs block to keep it safe.
Last character I picked up (and by far my favorite) was Leatherface. I specifically only play the butcher variation because it encompasses the fundamentals I learned with Quan Chi and has some of the forward advancing strings I loved when playing Kitana. Only downside to Leatherface is he's the high risk, high reward type. This means a lot of his good combos have gaps or can be punished for full combo on block. But if he hits you -- well let's just say I have a 36% mid screen combo (no meter) and a 38% corner bnb (also no meter). He has the damage, but it's getting it before you're dead from all the punishes that make this guy tough.
Last thing I'll say, and I honestly wish I had gotten this advice when I first started, is this: This game, as well as injustice 2, is about 20% character mastery. Yes, only 20 little small % of the game is actually mastering spacing, execution, timing, punishing, frame data, etc. for your character. Literally 80% of the game is knowing your opponent. You have to know the opponent in a couple of ways. 1, you have to know the opponent you're facing in the game. What are they're options? What should you do in certain scenarios? What's the best way to deal with XYZ and when is it time to punish or continue to block pressure. And my biggest demon -- what is a consistent block pattern for this character. Lastly, if you face a mixup heavy guy, what's the most damage each of his starters do (overhead or low starter). Those types of things determine how you block and such. Then there's the other half of knowing the opponent, and this half is way more complicated. This is knowing the human behind the controller. The good news is most humans all play games the same. They don't really live in the now of the game. They play it in their head and commit to things simply because they know how to, but not knowing if they should or not. However, playing the opponent requires a bit of testing and feeling him out for like the first 10 sec or so of the match. And if you've done your hw with your character, that should be enough time to formulate your first strat (yes, you will have composed multiple little strats throughout the first match, even throughout the first round you may change things up)
MK was always my favorite in the genre due to it's familiarity, and i definitely dig the fact you're coming 6 months after NRS' latest release to take a peek at what they did with the other series. That definitely shows some guts right there, so kudos!
Oh one last thing -- you're gonna lose. Like a lot ----- a lot a lot lol. I'm not the type to endure losing over and over again, so I was never capable of taking my game to the big stage. But those that can endure that chapter where they lose more than they win, but gain knowledge from each match they play --- those are the guys that we'll be watching on youtube in top 8s consistently for sure.
Anywho, longwinded Majik at it again (love this topic though lol). Hope that all helps in some way or another. I've been toying around with my past love affair recently (MKX still has it's grip on me), so ping me if you're on PS4 and we can run a few
