Shujinko all the way. As a player, we are introduced to all the events of his life, while Taven's quest was, at best, a couple months long - this is enough to make Shujinko infinitely more relatable and fleshed-out as a character, even though his personality is quite bland, honestly (but then, so is Taven's).
We see him play around his childhood neighborhood, then go to all the various realms, interacting with lots of characters and witnessing all the tangent events from MK since before MK1 in an interesting way that doesn't retcon nor disrupt the main narrative, but instead ADDS depth to it. He also serves as a way to finally introduce Onaga, who was hinted at in the previous game, and just how powerful and manipulative he is - as Shujinko is quite literally collecting elder god-grade tools for him throughout years.
Meanwhile, Taven's story is somewhat shoehorned in the lore and takes a central role without really enriching much of the narrative, basically functioning as a plot device to say "armageddon is happening somehow, someway". There is no tie to the previous game. They could quite simply say "the Kamidogu were the things keeping the realms stable and the One Being dormant and now they are shattered, which means ARMAGEDDON". Instead, they came up with a convoluted plot out of nowhere about 2 brothers and Blaze and whatever.
And, in a way, Shujinko is probably the strongest character overall, ever. He defeated Onaga, which possessed all the Kamidogu at the time, while Shao Kahn himself didn't dare fight Onaga 1vs1, even before he had the Kamidogu, instead having to resort to poisoning. And this brings another interesting discussion: how did Onaga get defeated by Shao Kahn in armageddon? And how did Shujinko, of all people, got imprisioned? Which mostly can be explained as bad writing.
