The Green Death.
Jo Grant's final serial. A good story brought down somewhat by the writer's problems integrating the story of the giant maggots and the AI together. These elements would have probably worked better as two separate stories.
I also think Jo's departure was sudden. None of the preceding stories had any buildup or mention of her desire to leave. That said the final shot of the Third Doctor just getting in Bessie and leaving was an excellent ending.
All the Classic Companions exits are sudden (With the exception of Teagan harping all the time about being over it). In the case of Jo Grant though, at least there was a 6 episode story where she fell in love and got engaged to give credibility to her hasty exit. Liz Shaw simply stopped showing up to the lab, and Jo showed up in her place, so, Liz didn't even get an exit.
One Day I hope to be the Man my Cat thinks I am
Where are we going? And why are we in this Handbasket?