Figma. It's not all that useful for actually mimicking a whiteboard, in the sense of drawing arbitrary shapes with a pen. It has a pencil tool, but it's not great. Fortunately, most of what we do is drawing rectangles with text in them and lines between them, and it's pretty good at that. Plus, it supports undo with cmd-z, which most whiteboard apps don't seem to.
+1 here. Figma's collaboration features are also just incredible so many people can participate simultaneously, and make copies w/ their own versions.
We've got ours also well setup with a big library of assets like buttons, and illustrations we've had made, and different example charts, so if what you're discussing is a new feature or landing page it's really easy to drag in a bunch of ready-made components to express your idea.