Muzzin is good in his own zone, yep! Holl, really? It's like saying Kulak says hello! Brodie isn't anything special in his own zone, typical leafs dman. Even there best dman is not known for his defensive plays. Muzzin is great tho.
Holl is pretty decent in all facets, skates well and has size (that he uses well). He performed really well in a top-4 role last season, and while there's always some uncertainty with late bloomers like him (1-year wonder and so on), he's definitely worth mentioning.
Brodie has also performed very well in his own end. He doesn't throw his weight around, but he is very sound positionally and has good stick work. He's far from being a liability in his own zone, and that alone will prove to be a much needed improvement over Barrie.
There's also Dermott who I didn't mention, but he's very good defensively, and he's not a guy you can push around easily either despite him "only" being 6'. He's probably going to continue improving over the next few years, since he's still very young.
Overall, while the Leafs don't have any d-man who are well known for their defensive plays (like Marino, Brodin, Pelech, Slavin, etc.), they definitely have a much improved defensive core. Some of their forwards have also shown some progression on the defensive side of things over last season, so it will be interesting to see how they fare with a stronger defensive core. I could definitely see them taking more risks offensively due to their stronger backend.