Figma is pretty good although for all types of design. It has real-time collaboration and perf is pretty good.

Whiteboarding specifically - don't know of any good ones. It should ideally be a collaborative multi user real time web app (websockets?), support drawing on top of uploaded images, have very large canvases you can zoom/pan, and run together with hangouts/Skype or have VoIP built in. Want to collab on building one? :)

I've used many Wacom tablets - even the cheap ones are good and pretty intuitive - no real learning curve after setup.