I was never really a fan of The Undertaker. That being said, his tweener and eventual heel run (1998) that included the Ministry of Darkness (1999) was one of the greatest characters ever.
I thought American Badass Taker was kind of lame, though his "Respect" angle was really funny. Turning heel by shoving JR's face in Vince's ass was one of the funniest things I've ever seen in wrestling. Continue on with beating down Maven, the Hardys, and pretty much anyone else.
Still though, I give the nod to the "Deadman" based off of 1998 and 1999. I mean, come on!: