I can't really single one out but I wouldn't be against any match if you make one.
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Shun Akiyama - Loaner, former homeless, obviously has Heat amps, crazy agility (even better than Majima's), can heal with Staminan like others. Also has power mimicry, can copy techniques upon seeing them once. He can do stuff similar to this guy.
Taiga Saejima - Former yakuza, a genuinely good guy framed in murder. He's more or less equal to Kiryu in skill, at least superior to him in use of raw strength. Can heal, amp himself, create shockwaves and tornados (Tiger Essence of Shockwave and not only that, Yakuza's strongest characters usually do them with regular strikes). Can jump really high and also has power mimicry.
Yoshitaka Mine - Kiryu's most powerful nemesis and his antithesis, essentially what Kiryu would become if it wasn't for love from his foster family (My favorite antagonist in the series). Is a very proficient MMA fighter and could **** up Kiryu badly, even if he lost (by lore he was the only one who could kill Kiryu, as his Kirin tattoo was symbolizing that). Can regenerate, has some really good acrobatics and has amazing stamina.
Masaru Watase - Similarily to Kuze, joined the business to fight strong guys and to build up his position with honor. Can make fire moves.
All from the more interesting ones, aside Kiryu and Majima obviously