OmniGraffle Pro
Used by 4 peopleomnigroup.com ↗

“For diagramming and communicating visually, Omnigraffle Pro is king.”
Paul Tweedy Interview, Senior Technical Architect, BBC · via Uses This 
“For wireframing, I typically use Omnigraffle Pro.”
David Sleight Interview, Creative director, web designer, tinkerer · via Uses This 