I was using Ice before the season started and Fire in the season, and also wondered why everyone was taking physical so just for interest sake I tried it and it boomed all the other elements out of the water, the pull from Cave In and combination of Battle Rage Bloodshed and Boulder Toss(Physical) creates a good synergy.
I use cold i find that best since it freezes and slows down mobs with iceblink gem, gives me a bit time before cooldown on leap kicks in so i can hop again, toss warcry and boulder and repeat xxxx times
And also cold is very good when u play 4 group fire is meh and so is physical.
I did a gr 65 lastnight with non ancient earth weapon and over 4 min left so i can easy top 70 with ancient weapon i think...
Not seen anyone else doing it but I'm playing Earth with Seismic Slam (Fire), just hit Para 600 and have already been doing 70 Rifts with a lot of lacking gear, no bonus str added via cube to any gear. I've not done my research and not even looked at any sites or played D3 at all since early S4 but I'm guessing the popularity of Raekor or Earth with Boulder toss shows it should be better, but this Seismic Slam build will go a long way above 70 for sure.