You can't, because the API of Blizzard's leaderboards doesn't offer any information about gear. You can only check the leaderboards in-game one-by-one. And even then you only have the highest clear per account, so if the #1 player cleared another GR with another set just one tier lower it won't show up. And from experience the ranks #2 to #1000 just copy #1, so it's only natural that leaderboards always converge to one set. Until someone starts using a different set and discovers that it's strong as well.
Diablorank.com used to have such a filter feature, but it's currently disabled. Going through all in-game leaderboards and taking screenshots is a lot of work and I guess it's not worth it right now to do this where ranks change on a daily basis.
And from experience the ranks #2 to #1000 just copy #1, so it's only natural that leaderboards always converge to one set. Until someone starts using a different set and discovers that it's strong as well.
which is quite frankly almost all the time for almost all the classes.
And just because of that fact you mentioned everyone talks the same about build diversity in Diablo. Cus they observe the leaderboards and only see the same builds used cus it's the strongest.
Crying out for balance.
While this might be true in some cases, there is still alot of balance (and therefore diversity) in the game that just gets ignored.
Just as an example the EQ Barbarian and Raekor/IK combo Barbarian probably is around 1-2 GR levels behind in power compared to the WW Barbarian. Still the whole leaderboard is almost entirely filled with WW Barbs exclusively. 1-2 GR levels is a pretty good balance in that example.
You can't balance all sets and all classes for the same GR level. That's just impossible.