Oathkeeper is actually the most common mastery here by far.
I wanted to do a proper breakdown of this, so that comment is a great diving board.
oathkeeper features 11 times in the list, more than half the builds … If you include the 15 Honorable mentions, 8 or 9 are also Oathkeeper, increasing its lead even more.
This is followed by Shaman at 8
and then Soldier at 7. Of those 7, 4 are warlords. Without Oathkeeper, Soldier is middle of the pack.
Necro is there 5 times, 4 of those being reaper and oppressor, who seem to have a great synergy.
Arcanist and Nightblade 3 times each
Occultist twice
and Inquisitor only once.
You need to go down to the second-to-last honorable mention to see the first (and only) demolitionist.
When it comes to damage type, Physical is indeed the most common at 9 builds out of 20, almost half. Then it dramatically drops off to 3 for both Retal (as a whole) and Vitality.
Lightning gets between 3 and 1 1/2 depending on how you count lightning retal and trozan.
The other spots are shared one each with bleed, Aether, Chaos, and 1/2 cold.
No Fire or pierce builds and no Acid build if you exclude Retal.
So, the 2 things that actually shamelessly dominate here are Oathkeeper and Physical damage. Oathkeeper, especially covers over half the entries, no matter how you slice the list, or if you include honorable mentions or not.