It would likely be something more than 12 tonnes. I only think Zygolo and Deino are heavier, but Zygolo’s huge tusks might be a disadvantage, so I’m going Deino

Woolly mammoth is big

I don't know enough of this to vote

^vote randomly then

Strength is hard to gauge since we don't have much on muscles, but I assume the Steppe or Columbian Mammoths would be able to lift the most

^^^^^Tusks are quite literally the main weapons of large proboscedians, Zygolophodons tusks are literally it’s main advantages. Not to mention is is the largest of this group, weighing roughly 16 Tonnes