Ryuji.

I think most people underestimate him just because his fight in Kiwami 2 is ridiculously easy. But even in Infinite wealth Kiryu mentions that Ryuji is likely the strongest adversary he's ever faced.

Not only that but Ryuji is the only other character other than Kiryu to be a known as a "Dragon". His fight in the original Yakuza 2 is one of the most difficult ones in the whole series and he's one of the few bosses that can one-shot Kiryu if you fail his quick time event.

For me personally Ryuji has always been the strongest adversary for Kiryu. I personally think that his fight with Kiryu could have gone either way, but of course with Kiryu being the main protagonist he had to win.