Whiteboard.Team
Whiteboard.Team is the online whiteboard that is my personal go-to. It is free and doesn't require a login if you can't be bothered (can save if you do login). But more importantly, the actual drawing tools available to you are some of the best. I also found that it supports copy and pasting in images which surprisingly few whiteboards seem to.
NoteBookCast
NoteBookCast is a good online whiteboard that is targeted at those who want to collaborate. It has in-built chat and you can create multiple pages. It also has a ruler tool which is always something I look for :-) The video I found here is in Spanish, but if only understand English like me then turn on captions and change the language to English in the settings.
Whiteboard.chat
If you want a whiteboard that gives each student their own board that you can see live then whiteboard.chat is probably the best out there. It's really well developed with lots in the free version and lots of help videos on YouTube + teams/google integrations. My only comment would be when you first load it up, it does look a bit complex! But this is only really because of the depth of material on there.
Explain Everything
Explain Everything offers something a little different again in the competitive online whiteboard field. Firstly there is a focus on recording. In Explain Everything you can create a video from your creation and edit that video as well. Secondly it is accessible by web, but also by app. And web versions on phones are often a bit challenging so this is a definite plus.
Whiteboard.fi
Whiteboard.fi is all about collaboration between the teacher and the students. The teacher creates a class where everyone gets an individual digital whiteboard. The teacher sees all the students' whiteboards in real time, while the students only see their own + the teacher's. It's an amazing piece of tech, however there are a few features sadly not included in the free version which would absolutely be what you want...
Zoom Whiteboard
I use Zoom whiteboard in many lessons. Often because I already have a zoom session open so I can use the annotation tool and therefore it's just one more click. But it's a great habit to get into, online whiteboards are just better than physical ones. Mainly I feel for the colour options, straight lines, 'undo' and saving. NB Some video features may be paid for.