The top opposition of each era for test bowlers

[Days of Grace]

I'm redoing my test bowler ratings and one criteria I want to include is a bowler's average against the top opposition of his time.
The problem I have is in ascertaining what was the top opposition for each bowler. Should all Shane Warne's matches against England count, or only those from 2001 onwards, when England was considered one of the top three or four sides in the world? When should matches against the West Indies cease to be counted?

So what I am asking for is what do you think was the top opposition of each era.

Any ideas would be greatly appreciated. Thanks.