If you would of asked 90's Deadman that would of been my answer hands down. Loved The Undertaker gimmick in the 90s, especially apart of the Ministry. But since we're only going from 2000/2001 onward. My answer is going to have to go to Big Evil. I actually enjoyed him as the American Badass. Especially when he was a heel. Current Deadman was alright at first, but wasn't as enjoyable as I once found it to be.
Big Evil by far. It was an awesome, yet believable transformation from such an iconic gimmick. When he went back to being the Deadman, it just didn't feel right (and maybe part of the reason is that they already broke the kayfabe mystique by doing the Big Evil gimmick in the first place). It's just felt like he's been playing a character and going through the motions ever since New Deadman.
Of course, my favorite Taker will probably always be Ministry Taker, but that's not completely relevant to the discussion.
The Biker gimmick hands down. The Undetaker was humanized and given freedoms he never had before allowing him to become a better performer in the process. Ministry Undertaker was cool too. The Undertaker in the beginning of his run as the dead man in the 90's was one of those once in a lifetime effective gimmicks that became larger than life because of how everything looked, sounded, and felt but when you really think of it he was just one of many gimmicks in a cartoon company catered to children. His gimmick was to scare children or something. They had a big fat Samoan guy playing a Japanese Sumo wrestler that sat on people, a Clown who was evil at one point, Lex Luger a muscle bound freak, right down the line to the plain absurd gimmicks like Mantaur, Phantasio, and Waylon Mercy. The Undertaker was the best of them all but he was just another cartoon freak of many. It took becoming the Biker guy to really bring the best out of what Mark Callis was capable of.
I like the Big Evil gimmick more than current deadman that was the period when he became the worker he is now plus it was in that era when Smackdown was awesome and he was having them great matches with Brock and what not.
Big Evil but I think as a character I've completely lost interest in him since 2004, I mean 1990 to 2003 I like all the incarnations from Original, Ministry, ABA and Big Evil but once that ended I just felt since 2004 he's been a poor mans version of his gimmick in the early 90's.