I think a problem with the third party sites is a lot of them use arbitrary rules.
Like no seasonal event achievements. No legacy achievements. Saw one that didn't include PvP achievements.

Basically saying to us "All this work you did doesn't count." And that's just unfair to the people who put in the work.

I think the best one I've found was a Japanese one, but I don't know what kind of system they use since it doesn't seem to include everyone in their top contender list.