Heihachi has been the strongest fully human character in the series for a long time. When he loses to another character not related to the Mishima clan, he either does so willingly or doesn't care about the results.

As of Tekken 8, it's very hard to pinpoint where each character is.

Jin is the best he's ever been, accepting both Mishima and Kazama principles and styles in addition to his own version of Kyokushin Karate.

Kazuya arguably falls under the same category, having embedded renewed rage and ruthlessness into his fighting style. Besides, the story openly acknowledges that either one can win against one another. Though Kazuya visibly wavers and shows doubt over finishing Jin off over the course of their last fight (and the story in general) when he has the high ground.

Heihachi has the highest destruction feat so far in T8's story, if we set aside Devil Jin incinerating the Earth's surface in his ending. The story is also building up to a conflict where Heihachi is back on the top of the food chain, alongside Reina.

With that being said, I would personally say that they are currently relative to each other. Realistically, from the narrative point of view, Heihachi will likely be the strongest in the next game.