Den of Geek asked me to write an article ranking every single companion departure ever, so I did. It sounded fun, such a gratuitous piece of clickbait but simultaneously something that could show a different perspective on the history of Doctor Who.
And so it proved. I simply listed every companion who left (which included characters who filled that role for one or two episodes on the basis that it would be interesting to write about them too, even if they only filled the role for a limited time) and wrote about their exit, then ranked them afterwards. Don't get too distracted by the rankings, even though they're right.