>>42397096
Here are most of the ones involving Celestia and Twilight that I put on any of my bookshelves. Apologies if I don't remember them all too well. Short descriptions because I'm tired and also exactly hitting the character limit.
https://www.fimfiction.net/story/246196/basking
3.1k words
>Celestia's too sick to even get out of bed, and Twilight's worried.
Celestia's out for most of it, but I really like the descriptiveness when narrating her sickness or Twilight's distress over it.

https://www.fimfiction.net/story/33777/dictated-not-read
3.2k words
>Twilight tells Celestia about a bad Hearth's Warming gift she once recieved as a filly, but really, her letter's about much more than just the gift itself.
Sadfic focused on Twilight's relationship with her parents, and perhaps a little bit about her relationship with Celestia as well by the end. I really liked it.

https://www.fimfiction.net/story/577870/the-foal-thats-crying
5.3k words
>Over the centuries, Celestia had developed a habit when selecting the student she'd personally oversee. It was always the one she found crying.
I couldn't remember much about this fic at first and had to skim it, but was quickly reminded that it's just because it's a collection of scenes not in chronological order, instead of one linear neatly-wrapped narrative. Added it to my list because it showed some pretty good emotion in there I think. Only a brief mention of Twilight but included anyway.

https://www.fimfiction.net/story/81525/the-lessons-of-eternity
4k words, an angsty post-breakup exchange between Twilight and Celestia with a side of angst about immortality, aged in a casket since 2013. Not everyone's cup of tea, but if it is yours, then you'll find it to be well-written.

https://www.fimfiction.net/story/229065/forever-and-again-and-again
4.3k words
>Celestia shows Twilight her collection of older stained glass windows depicting ponies that may no longer be around, but whom Celestia will never forget.
This one earned its spot on my favorites, and I say this as someone who prefers their immortalityfics with more angst (making me the opposite of the author). I don't have to reread a single word to remember why I like this one.

https://www.fimfiction.net/story/215912/passing-the-time
5k words
>Twilight messes up a teleportation spell and now her and Celestia are stuck in a pocket dimension for a week, with nopony but each other to pass the time with.
Cute little fic where they hang out.

And a couple of shitpost fics, about 2k words each:
https://www.fimfiction.net/story/362659/friendship-hate-mail
https://www.fimfiction.net/story/299254/i-have-this-friend
And a longer (4.6k words) but still pretty shitposty one where Twilight discovers her friends apparently think she's been dating the Princess this whole time. Despite the silly premise there's actually a pretty good bit in there where she explains their actual relationship.
https://www.fimfiction.net/story/239743/misunderstanding