Inspired by a random YouTube user commenting on this Deathly Hallows clip:
Does anyone know what memory Alberforth thought of in order to create that huge patronus? AeroRomero
We know that casting a Patronus
"[..] will only work if you are concentrating, with all your mind, on a single, very happy memory." Harry Potter and the Prisoner of Azkaban, chapter 12, "The Patronus"
I only remember one of these memories actually being described, namely Harry's one, which is alternatively thinking about his parents or Ron and Hermione (well, technically that makes for two memories).
As pointed out by Mithrandir in chat, this quote indicates that Snape probably thinks of Lily:
“But this is touching, Severus,” said Dumbledore seriously. “Have you grown to care for the boy, after all?”
“For him?” shouted Snape. “Expecto Patronum!”
From the tip of his wand burst the silver doe: she landed on the office floor, bounded once across the office, and soared out of the window. Dumbledore watched her fly away, and as her silvery glow faded he turned back to Snape, and his eyes were full of tears.
“After all this time?”
“Always,” said Snape.
Harry Potter and the Deathly Hallows, Chapter 33, "The Prince's Tale"
Some others we can try to guess:
Umbridge is likely enjoying the Muggleborns' treatment under the dark days of the Second War, with regards the following quote and this answer (as well as other posts on this thread):
The moment he had passed the place where the Patronus cat patrolled, he felt the change in temperature: It was warm and comfortable here. The Patronus, he was sure, was Umbridge’s, and it glowed brightly because she was so happy here, in her element, upholding the twisted laws she had helped to write.
Harry Potter and the Deathly Hallows, Chapter 13, "The Muggle-Born Registration Committee"
- Tonks' Patronus took a wolf shape due to her love for Remus, so maybe she was thinking of him, and reversely.
Then there are a couple more which I just don't know: Dumbledore's Army members, Abelforth, Lupin (before he met Tonks), etc.
What do we know from various sources1 about what memories people thought of to cast their Patronuses? What's the complete list of people whose memories we know and their memories? 2
I'm looking for explicit statements, or at least quite educated guesses, rather than just "Thinking about your loved ones seems to work, so maybe James was thinking of Lily".
1 Let's say that includes Pottermore, books (as well as Fantastic Beasts and the like), video games, movies, JKR interviews, DVD commentary.
2 For this question not to be too broad, let us consider the list of Patronus-casters from Pottermore. I'm mainly interested about Order of the Phoenix members, but bonus points for Dumbledore's Army members.