Kimimaro has impresive control of his octeoblasts and octeoclasts which enable forms of regen, however-I disagree with your criterea to what makes a unsurpassed regenerator.
-In my opinion the best regenerator is able to regenerate after the most damage caused.
-hashirama- is all hype so we don't know
-kabuto/orochimaru have incrdible regen abilties via oral rebirth, practically any damage done can be ignored with this justu. ( the justu doesnot require handseals) -Orochimaru looses limbs, gets cut in half and laughs becaues of his oral rebirth. - Now, its not passive but it still regeneration so I dont see why you wouldnot count it.
-karin- healed being stabbed by spiral zetsu
-Kisame- healed w/samahada - having his entire chest cavity open
-tsunade- cut in half, and healed, stabbed multiple times and healed.
-compared to top teir healers kimmimaro is fairly poor- show me a panel of him being cut in half and surviving and then he will join the ranks of top teir healers.