Of all the lists I've made for this community, I think this is my favourite one... hee hee. All of these fics are waay AU long, majorly, plot-driven fantasy lands, which incorporate a ton of 'ships each. Of course there are het 'ships within each of the fics *if you squint hard enough*, but the main parings are all slash (and I'm not even going to try to be anal and list every 'ship within, 'cuz I know I'll miss some), so that's what I called the list slash multi-ship. I think most of thse fics are underappreciated, so give 'em a chance, check-em out and send lots of positive feedback to their authors, cuz, well, they deserve it!
1.
Lifting the Veil Series by Serafina
(Main: Wes/Lindsey; Also: Wes/Angel, Angel/Doyle, Angel/Xander, Graham/Oz, Buffy/Spike, Cordelia/Gunn, Willow/Tara, et al.)
Lindsey and Wesley save the world and each other with magick, a little help from their friends (including some interesting and unexpected Btvs/Atsverse characters), and and a mandate from the powers to do battle with some of the coolest original characters in the fandom. I don't know where or when I first read this series, but it's completely 'enchanting' from beginning to end. I don't know if I've ever seen such rich and complex Wesley (well maybe TBQ's wes) or Lindsey characerization that I like more.
2.
The Soft Insanity of Time by Sajinn
(Main: Angel/Spike; Also: Gunn/Cordelia, Wesley/Fred, Xander/Oz, Buffy/Lorne, Willow/Dawn-ish, et al.)
Fighting for the powers is never easy and taking down Wolfram and Hart is a long, hard process full of losses and changes that NONE of these characters ever would have imagined or wished upon themselves. Then again, conflict often makes for great family bonding and this fic takes that idea to it's extreme and watching this 'family' emerge and blossom in the face of adversity is a truly unexpected joy.
3.
Kindred Souls by Jericho
(Main: Xander/Spike & Xander/Jesse & Jesse/Lindsey; Also: Giles/Angel, Ethan/Giles and some squicky pairings including abuse & incest... really, read the warnings)
This one is seriously underrated, but I suspect that's because of the main child-abuse plot (no, none of the characters you love are abusers, but they do get bashed about a bit). If you can stand that sort of thing, take a peek cuz it's worth it.
4.
Thralls by nei'chan
(Main: Angel/Xander/Riley/Graham + Doyle + OMC; Also: Spike/Oz/OMC, et al.)
Maggie the initiative Bi-atch has no idea what she's playing with when she experiments with some supernatural mojo. Long story short, Angel becomes some sort of almost invincible Vampire king and local power-structures get re-aligned. (This was going strong last year, but got stalled. I still have hope that she'll pick it up again, but I wouldn't bet on it).
5.
Bent Justice by Kat
(Main: Angel/Spike; Also: Wesley/Gunn, Angel/Penn, Spike/Faith, et al.)
OK, so this fic wasn't underappreciated (really, I think inuits who've never seen an episode of Buffy in their lives managed to get talked into reading this), but it's not one bit overrated either. There are really no words to express the hottness of this Spangel Human AU prison fic.
Honorable Mention:
Searching for Oz by Matthewtime
(Main: Oz/Surprise; Also: Angel/Spike & Xander/Doyle)
This one didn't make the list because it's short and only involves 3 pairings, but I love it to bits and read it a few times a year, so I had to mention it because I love it so much... really, it's a quick read, pop on over and check it out. I promise it'll make you laugh :-)