Undertaker, overall.
Both had gimmicks that should've been dead in the water. Undertaker's brother is a character that's really hard to live up to. Honestly, that easily could've stunk the joint out but Kane pulled it off. Hard shoes to fill, so he deserves props for that.
I think he was physically more impressive. I don't think he had the same psychology and ability to put together a match. He was good, but he probably wasn't capable of having GOAT match discussions like Taker/HBK.
He did deal with some baaaaad booking and came through it, +1 one for that too.
Taker is Taker so no knock on being #2 to him.
OG Kane is one of my favourite monster characters of all time