The Tag of "The Wedding of Sarah Jane Smith", one of the last filmed appearances of David Tennant as The Doctor:
The Doctor: Don't forget me, Sarah Jane.
Sarah Jane Smith: No one's ever going to forget you.
The look on Sarah Jane's face as Peter renounces his agreement with The Trickster is both this and a Tear Jerker.
Seeing the Brigadier in "Enemy of the Bane" after so many years. The looks on Nicholas Courtney and Lis Sladen's faces must be seen to be believed. Sladen and Courtney had seen each other somewhat regularly at conventions, but it's entirely possible that the last time we saw Sarah Jane and the Brigadier onscreen together was the last time Sarah Jane and the Brigadier saw each other. Moves into heartbreaking Tear Jerker territory, though, after 2011... and yet how fitting that it's these two who were reunited so shortly before we lost them both for good...
The Brigadier: (reverently) Sarah Jane Smith.
Sarah Jane: (warmly) Hello, Sir Alistair.
Any time Luke and Sarah Jane are reunited. Hell, just the entirety of Luke and Sarah Jane's relationship, period.
The moment in Death of the Doctor when you can almost see Russell T Davies think "to hell with it" as he dives into the Continuity Porn:
Sarah Jane Smith: There’s a woman called Tegan in Australia, fighting for Aboriginal rights. There’s Ben and Polly, in India, running an orphanage there. There was Harry... oh, I loved Harry. He was a doctor, he did such good work with vaccines. He saved thousands of lives. There was a Dorothy something. She runs that company, A Charitable Earth. She’s raised billions. And this couple in Cambridge. Both professors. Ian and Barbara Chesterton. Rumour has it, they’ve never aged. Not since the sixties. I wonder... echoes of the Doctor, all over the world. With friends like us, he’s never going to die, is he?