Tbh, after the ferra patch, its Mk1. I was very skeptical about kameos, as I hated the assists on towers of time in MK11, but ended up loving them. It also helps they fixed almost every gripe I had with MK11's gameplay (limited combos, some characters obviously much better than others for no reason, instant fatal blows, the meter system) AND MKX's (too many hard to blockables, subpar balance).
Mk9 still has my favorite art direction (no "stage roster" is even CLOSE since then) but Mk1's gameplay is peak.