Assuming we're limiting things to Part 1, then:
Tier A
Raoh
Toki (pre-illness)
Kenshiro (post-resolve boost) = Toki (Sick)
Tier B
Souther
Juza
Tier C
Fudo ~ Ryuga
Shu
Kenshiro (Pre-Raoh fight) ~ Rei ~ Shin
Tier D
Yuda
Jagi
Kenshiro (pre-story)
Even though it was stated Toki could've beaten Raoh if not for his illness, it wasn't in terms of raw power as Raoh was stated by both Toki and Kenshiro to be the strongest man in Hokuto Shinken's history. Kenshiro was shown to be equal to a sick Toki just before Toki VS Raoh and Souther was stated to be the strongest Nanto master which should include the Goshasei such as Juza, not to mention having arguably greater speed than Kenshiro, though clearly lagging behind the Hokuto brothers a fair bit in power.
Fudo is stated in guidebooks to be on a similar tier to Rei and the like, which seems fair as we have little basis for Raoh being that strong back when Fudo scared him and he only lasted so long in their fight due to resolve. Ryuga is a tough one to place. Guidebooks place him below Shin and Rei, but that seems very questionable. Given how Raoh didn't immediately dismiss him fighting Kenshiro, I'd say he ought to be at least above Rei. Shu is clearly above Rei and Shin from his fight with Kenshiro and Kenshiro prior to fighting Rei should be either equal to or slightly stronger than Rei based on how they're presented together as well as Kenshiro's statement in Part 2 about surpassing stronger opponents through Hokuto Shinken (where Rei was shown) likely makes them at least equal all around. Shin in turn should be about the same level going by guidebook entries and Bat saying a single move could determine the entire battle between Kenshiro and Shin.
Yuda is clearly the weakest of the Rokuseiken (bar Yuria) with him still losing to a near dead Rei even after all his cheap tricks and despite Nanto Kokakuken arguably being the most skillful Nanto style. Jagi, despite being implied through his confidence and Kenshiro's initial worry to be above the Ken that lost to Shin, was clearly outmatched by Kenshiro during their fight. Could even possibly place him a tier lower given guidebooks place Yuda a tier above him.