I find this hard to get my head around.
My personal favourite of all time is HBK, and so is millions of others, but lets be honest, HBK hasen't achieved what Undertaker has achieved:
Wrestlemania Streak
18-0 at WrestleMania and like Arn Anderson said on RAW, that's like a team winning 18 Superbowl's in a row, that's an unthinkable achievment, something that sends chills down my spine when i think about not just the streak but all those who fell victim to it- HBK! Ric Flair! Edge! His Wrestlemania opponent HHH!, all guys who are Hall of Famers or certain future Hall of Famers and not to mention the dozen or so others. No one will ever repeat that or even come close to it in my mind.
Championships
I don't know how many WH/WWE Titles Undertaker has won but certainly i know he's won alot, HBK was only champ 4 times and he's regaded as maybe the best ever (i know he's done so much more but that's a completley different matter), HHH has won 13 and Flair 16, but my point is that Undertaker can certainly match that and he's always a contender for a championship when he's not injured.
Other Achievements
He's won Royal Rumbles, Elimination Chamber matches and not to mention his personal matches such as Hell in a Cell and Buried Alive matches, Undertakers done all there is to do in the WWE, he's won all these types of matches, beaten all of the greatest superstars to walk through the door and there are still a few years left for the Undertaker to achieve more.
Reasons why Taker may not be the greatest ever
There isn't alot to complain about Undertaker but there are some things:
His Character Personality- When Undertaker steps into the ring, he does what needs to be done, wins the match then leaves, he doesn't talk about it much and he doesn't really feel the hate of his opponent that his character is supposed to personify. Take his Wrestlemania opponent HHH, one of the greatest ever (some may say the greatest ever), one of the reasons he is so popular was because of his character personality and mic skills, when he was heel, people loved to hate him because of what he said on the mic and when he was face, he was loved by what he said on the mic. His actions with HBK as DX was loved by the WWE fans and Undertaker just dosen't bring that excitement on the mic that most of the superstars bring. I know his character is supposed to be dark and distant, but it isn't scary or intimidating like its supposed to be, it's just a bit boaring if im honest and that in my opinion is Undertakers only downfall.
Obviusly im cluching at straws with his weakness but i feel, yes the undertaker is the greatest ever because of his countless achievements, he's not my personal favourite of all time but he is the greatest ever, no doubt in my mind.