you cant compare champions like that. the stats in this game are too varied. that's why you both have different opinions. to your friend raw hp might be what makes a tank. to you it might be something like spell shields and basic attacks. vayne for example is strong against pure hp builds. mundo has some amazing regeneration and his stats scale from HP so he can do damage while just building pure hp, but if he goes against vayne he's instantly wiped off the field. zac has regeneration and all of his ability scale from AP while being almost resourceless just like mundo.
you have to specify your scenario.
the best generalist tank is sion. the best damage mitigation tank is alistar. the best aura tank is amumu, the best distraction tank is singed. the best harassment tank is nautilus, the best damage soak tank is rammus, the best crowd control tank is blitzcrank, the best utility tank is maokai, the best offensive tank is leona, the best defensive tank is shen, the best anti-magic tank is galio, the best anti-physical tank is malphite, the best flanker tank is poppy, the best mobile fortress tank is sejuani, the best engage tank is zac, the best dueler tank is jarvan.
those are the only champions classified by riot as tanks in the client.
in the case of catfish breath - he's classified as a support. my opinion is that taric, braum, and rakan are better at tanking than kench. the reason being is that kench's bubble shield does not regenerate a large amount of HP or prevent damage for more than 2 seconds. more so his abilities do not scale with armor, magic resist, or HP. at least taric and braum scale with armor. i however cant remember if kench has an HP variable in one of his stats or not. i feel like he might.
regardless - due to the simple fact that kench is a support first and foremost he's going to automatically fall flat in the tanking aspect. though as a support that shields his allies with his own body he brings a lot to the table. if your friend means tahm is the most hp regenerating support then yes he's correct.