Really, this is an impossible task. You can make the case for no.1 and 2, but after that it is really subjective.
Using the rule 'an all-time great in one era would be the same in another era' I am putting those who were considered as near unanimously the best of their era. To me if within the two decades you were playing, you are recognized as the best that puts you in a special category.
1910-20s Jack Hobbs (followed by Sutcliffe)
1930s - 40s Bradman (followed by Len Hutton)
1950s - 60s Sobers (followed by Graeme Pollock)
1970s - 80s Viv (followed by Gavaskar)
1990s - 00s Sachin (followed by Lara)
2010s - 20s Smith (followed by Kohli)
So, excluding Smith because he is still playing, the top five should be among Hobbs, Bradman, Sobers, Viv and Sachin. Out of that list, Bradman is no.1 for being so ahead and Sachin in no.2 in my book for his technical and statistical near perfection over the longest international career. The remaining three would be tough to separate but I would go with the order of Viv, Hobbs and Sobers.
The remaining 6-10 can include Hutton, Pollock, Gavaskar, Sutcliffe and Lara. Pollock and likely Sutcliffe would be removed for not playing enough tests. It would then be a tight tussle between Aussies Greg Chappell, Border and Ponting for the remaining two spaces. Chappell and Border to me gets extra points for being in a tougher era. I am going for Lara, Hutton, Gavaskar, Chappell and Border.
Final list;
1. Bradman
2. Sachin
3. Viv
4. Hobbs.
5. Sobers.
6. Lara
7. Hutton
8. Gavaskar
9. Chappell
10. Border
11. Ponting