Defensive Build or Power build [Assassins]

Remember that this is in a 1v1 scenario.
That's why in 1v1 Joust, people almost always counter build and take some defense.

However, in Conquest, you cannot focus only on 1 type of protection.
In that example, when Defense Thor had Bulwark of Hope against magical defense, the only reason he live was because of the passive proc.
Otherwise, it's a complete waste of item slot for that 1v1.

While Power Thor was good against any enemy, just a bit less against a physical tanky target,
Defense Thor would be only effective against squishy enemies.

In most cases the most dangerous targets are the squishy ones. remove them quickly from a teamfight and the enemy team "lost" their damage, which ends up into your team (assuming you didn't lose your damage dealers) win the teamfight.