Basically Sub's D4 is the fastest d4 in the game and it's safe on block so you can use it as a primary footsie tool up close. You can use d3 or d4 to get out of pressure or to counterpoke. A D4 on hit grants enough advantage for you to dash up and do 21 pressure. Also never use Sub's D1 because D4 is just better.This looks quite important. Normally I just go by experience, find what works, and remember the recovery times on everyone's movesets. When facing a solid opponent, I joke often, "wow, this guy must've studied the frame charts. Geez." But now I see how important it is.
Given as far as I've gotten in MK9, however, I don't know what to do from here. I've either read attacks as simply "safe on block" or not, and which comes out the fastest.
You can make any string safe by cancelling into clone but some stuff like 214 clone (IIRC) can be punished by fast advancing specials like Scorpion's spear. There is a gap you have to worry about between 21 and 2, and your oppnoent can armor through it. So it is 2,1 (gap) 2 and they can armor in between, so watch out for that especially when setting up the 212 clone trap in the corner.