The concept of Kane when he first debuted was dark & awesome & I loved it. Masked monster who appeared scarier than the Undertaker at first. Kane definitely has done well on his own and has still managed to keep the reputation of being the Big Red Machine. Again he debuted for the sake of doing something above & beyond with the Undertaker so since his debut he already played second to the Undertaker. In his defense, Kane has managed to continue his work in the ring far beyond the Undertaker's capability (once a year
) and that makes him hardworking.
Undertaker however has wowed generations with his many gimmicks. He started with the Deadman gimmick which continued into the Ministry of Darkness where he only got darker & more sinister. He then returned as a Badass Biker straight out of Houston, Texas. Then he continued that into the Big Evil heel gimmick & finally back to the Deadman. He did all this while juggling world titles & putting on awesome matches. Undertaker has had some of the best feuds in WWE history. In my top 10 for sure.
Gotta give this one to Taker