The difference between Zombie Liu and Revenant Liu is that Zombie Liu does not have Liu Kang's soul. He's essentially a vessel that contained Liu Kang's fighting skill but none of his conscious thoughts; he simply has the single instinct to kill everyone who was a threat to Earth Realm (or perceived as a threat by Raiden, who resurrected him).
Revenant Liu, on the other hand, does have Liu Kang's soul. He retains all of his memories, but his soul has been corrupted so that he is evil and does things that normal, human Liu would not, such as answer to Shinnok, try to kill Raiden (although he did try to do that in MK9, although for a much different reason), and support the conquering of EarthRealm. Although now that Shinnok is dead and he is co-ruler of the NetherRealm, it's unclear how many of those things he'd still do. But it is clear that he is still dark, and capable of things his non-corrupt counterpart would not do. As opposed to Zombie Liu, he is obviously capable of rational thought and must be pretty cunning to rise to leadership over many other revenants of similar power and status.
As for the strongest version of Liu Kang, I can't say for certain, due to the limited knowledge the audience has about Rev or Zombie. However, overall, the original, human Liu Kang has shown time and again to be one of the strongest characters in the MK Universe.
He defeated Shang Tsung, Shao Kahn, and (although it may have been retconned at some point) Shinnok throughout the original MK games. All of these bosses are among the most feared and powerful characters in the MK universe, and being that he beat several of the more than once, I'd chalk it up to him actually being more powerful than them rather than him just being lucky.
Revenant Liu Kang, on the other hand, seems to be more manageable, even by characters of seemingly lesser skill. Although I feel bad for using MKX's somewhat sporadic fight scenes as an indication of a character's power. However, personally I feel that with his soul corrupted, part of what makes him Liu Kang is repressed, and he isn't 100 percent on par with human Liu.
Zombie Liu is out, for me. Liu Kang is a smart fighter, and without his conscience I don't think he stands much of a chance. His body is weak, as he is literally a walking corpse (as opposed to revenant Liu being completely physically restored). And while Rev Liu has his mind partially bound, Zombie Liu doesn't have a mind at all.
Overall, I think the biggest difference between the three versions of Liu Kang is their minds, not their fighting abilities. And I think the original, human Liu Kang's mind, free from anyone's control, is what made him the champion of Mortal Kombat, and one of the most powerful warriors in the universe. So, my two cents is, the strongest version of Liu Kang is his human version.
