Lucidchart has been my go to. Allows multiple users in the same whiteboard at once. Legible text as opposed to whiteboard scribble. Easily rearrange block diagrams. Permanent record of a diagram session that can be thrown in a design doc.

Excalidraw is killer. We use it extensively at work

I have a second webcam pointed straight down at my desk, and scribble with a pen on paper.

Frankly, this works better than any of the digital tools I tried, despite not being collaborative. (I tried a number of them, and have both a drawing tablet and a tablet pc)

Miro.com is a game changer.