Whiteboard + draw.io for component, flow and class diagrams (if it comes to that level). Whiteboard for UI. If I have a dedicated UI dev, they can take a pic of my whiteboard sketch and build a static prototype faster than me attempting to draw it in some legit fashion :)<p>Quick static&#x2F;stubbed out prototype app in some cases with whatever tech is appropriate (for mobile been digging ionic quite a bit lately for prototyping).