It can be anyone from Fury right down to Bam. Only bouts where they've successfully defended their world title (winning the title doesn't count).
How are you counting unification bouts? Those are both defending an existing title AND winning an additional one? And are you talking about defences in their current division and holding their current belt only? (I'm guessing, purely for example, you'd be excluding Usyks cruiser achievements even though he's a current world champion at heavy).
So winning any other belts along the way is permitted but only in that weight class. & yes only Usyk's heavyweight reign.