As we all know, Spander writers are insanely fond of getting angsty (but also having, inevitably, happy endings) and over the years, I've read a whole lot of angst-fic, but these are the ones that I always go back to.
1.
Long Time Gone by Yindagger
'Everybody and their mother' has read and recced this fic and... well... none of them are exaggerating in the least. This is the ultimate in Spander angst. Xander left Sunnydale for a nice normal life, only to have his wife and unborn child murdered. The telling of this particular tale is only the beginning of a very angsty and well plotted ride. Really, if you haven't read this yet, don't even bother with the rest of the list, just click and start scrolling... it's that good.
2.
Repossession by Lazulikat
This story should have been called 'Spike and Xander's Rollercoaster of Angst'. About every 10 or so chapters, you think, "Whew, the boys sure have been through a lot lately, now I'm sure she'll let them settle in for a nice happily ever after ending" and then BAM some other crazy sh*t happens that's even more bizzare and angst making than the last! This fic is beyond emotionally exhausting and I think it should come with some warning about NOT reading it in one sitting (because boy might that be hazardous to your mental health), but it's definitely, I'd say, one of the cornerstone fics of this 'ship.
3.
Twilight Time by Shadow
Abused Xander and Chipped!Angry! Spike try to make it work and solve mysteries at the same time. And seriously, you'll never have any cred as a Spander 'shipper if you haven't read this already, so if that's your bag, get to reading because this one takes a while.
4.
Reconstruction by Wordsmith
Even with the Buffster dead, the Scoobies and AI gang, lead by Xander (of all people), must forge ahead and save the world. The best part about this fic is that it's just the first in a phenomenal unfinished (but long-enough to complete a few arcs and leave you semi-satisfied) trilogy that will leave you begging for more. There's plenty of angst here (especially in this first part) and the best bit is the author's totally yummy notion of Dom!Xander.
5.
Back With Me by Alexandria Brown
Spike & Xan are having a hard time. Spike angsts about it all, calls for help and winds up with his problem solved, but having to take on his saviour's angst in the process.