The reason I've never done recs here before is that I usually have this automatic assumption that nobody is reading anything I write, ever. A lot of things I assume don't actually reflect reality, though, and lately there is more evidence of existant readers, so I'm going to try my hand at pointing you in the direction of good fanfiction.
It's all B/A for now. I mean, it's for Fluff! What else would I do? Also for the sake of Fluff I'm putting one of her stories at the top here, even though she's already read it, and I'm emphasizing it right now for everyone else so that you go there and give her reviews. She likes reviews! Give them to her! And wish her a happy birthday dammit!
I tried to provide a nice combination of deservedly well-known stories, and the ones that I personally feel need more attention. Also as a disclaimer I have to tell you that I left out some of my favorite writers. Fear not, I will continue to fret over this until I do another rec list.
Without further babble...
Exodus Laughing by Fluffernutter
There are a lot of stories by her I could have chosen, and I think you should follow links until you read/review them all, but this is a recent one that I found especially uplifting and deftly written. Plus, Fluff wrote it for me, isn't that grand?
La Mariee by Maren (Adult)
Probably the first fanfic I fell in love with and still at the top of my list. I've never spoken with Maren, but she's an astonishing writer.
Built to Last by Vatrixsta Cruden
One of my favorites from the last IWRY marathon. We get to see a happy Shanshu ending and also to compare it with the canon story and the potential future.
Love Lies Bleeding by Dark Star (Adult)
I read this before I knew who Dark Star was and it's still the one that sticks with me. It's a Buffy/Angel you don't usually see, and it hurts bigtime.
Echoes by Kita (Adult)
Sublime was the site where I did a lot of my early B/A exploring, and when I returned there recently I discovered that some of the authors of the stories I loved are people who I now actually know. Oh hai! Man I am seriously favoring the angst on this list, what's wrong with me. Sorry Fluff!
All Souls' Day by Spiralled (Adult)
There we go. This one makes me happy inside all the way. All the love, the affection, and the vampiric sexiness; none of the anger and loss that so often accompanies post-NFA reunion stories.
Courting Sin by Leni
If you've read any Buffy/Angelus fiction at all, you've read this one. If not, YOU'RE WELCOME. If not for the day I randomly found this story I might not have talked myself into getting a fanfiction LJ at all. It's true.
Wish Your Heart Makes by Violaclaire
In my opinion this is something that happened in Season One canon and was accidentally left out of the show.
I-55 by Yahtzee
You know Yahtzee cuz Yahtzee wrote "Phoenix Burning". This one's a lot shorter than that and I think I like it even more, but not for happy ending reasons.
Finis Vitae Sed Non Amoris by Taaroko
There's a lot in this story that you don't usually see from B/A writers: Buffy's time in Heaven, the intricacies of the vampire mythology, Buffy coping during Season 6, and the unseen meeting. Okay, so you do see the last one a lot, but not often in a context like this. Very worth your time.
Texting Under the Influence by Sugarless5
It's been said that B/A resists humor, and I think there's some truth to that, but I found this little gem recently, and...I don't know, I just love drunk Buffy, is that wrong? Short and incredibly sweet.