In order to pick the greatest allrounder, you need to look at each candidate and ask could they be selected as purley a bowler and purely a batsmen? That would eliminate the players who are good at one aspect of the game and not the other.
Kallis would not be selected purely on his bowling. He is a handy bowler, but nothing more. He is therefore a batmen who can bowl, and should not be considered the best allrounder. He could be considered a great great batsman though who also bowled a bit.
Sobers also would not make the team purely on bowling. Also a handy bowler, he could probably make a world XI side purley as a batsmen though.
Miller would be selected on bowling alone, and probably be selected as purely a batsmen as well. Remember, his statistics lie a bit, and he was one of the best batsmen in a team full of brilliant batters. This guy is a great allrounder and good at both aspects, so he must be considered. Not to mention the fact that he didnt get to bat on the flat wickets of today with a thick modern bat!
Botham, hed make it as a bowler easily, its hard to say though if hed make it purley on batting though. He is matchwinnning with the bat though, and could have made the team in that regard so he can also be considered. But in comparision to the competition i would rule him out, matching winning does not equal consistancy.
Khan could probably make it based on both his talents. He should be considered. Though not a great batsmen, he was certainly very good.
You can apply this kind of anaylsis to other players. Based on my thoughts, Khan and Miller are the two leading canditates for best allrounder, based on the fact they could do both bowling and batting equally well, and very very well at that! However i wouldnt class either of them the best, i would rather have tiers of greatness. With these two standing at the top.