The number one toy that my girls turn to again and again is a piece of bedsheet which my husband tied some knots on!
That can play tug of war for hours and growl at each other, trick each other and chase each other
When we only had one dog, it was her favorite toy as well, as long as we had the energy to play tug of war with her - no fun alone!
If she was to occupy herself for a while, we would give her a chewing stick made of ox hide, a nice big one that would be a challenge to her - that was a hit as well.
Now that they are 2, even if we give them one each, the funniest thing is to trick the other ones stick away