Via Radio Times comes this piece about five great companion exits. I mostly agree. However, in the case of Astrid Peth (Voyage of the Damned), while completely fine in terms of “great” and “exit”, it doesn’t quite fit “companion”. Sure, it was affecting, and it affected the Doctor a great deal, but Astrid belongs in the should-have-been category along with Lynda With A Y (Bad Wolf/The Parting of the Ways) and Rita (The God Complex). These are people with whom the Doctor felt an instant affinity and to whom he wanted to show all of time and space, but their lives were snuffed out heroically before it could happen, even before getting to step inside the time machine. There are probably others, but these are the ones who come to mind now.
Nonetheless all of these had me bawling my eyes out. Even Adric, when I was a lot younger than now, and I didn’t even like him so much.