Ask HN: What Is Your Favorite Online Whiteboard App?

Hi HN! Where I work, we have a lot of engineers that work remotely and in other offices. Even though we make a good effort of being remote-friendly, I often feel something is still lost in the distance. I want something that can come close to the experience of people standing at a whiteboard. I've looked at a few different ones, but haven't found anything that seems good for realtime collaboration, or works on mobile devices.

My question is

What does HN use for remote whiteboarding?