Hard to decide what to pick ... but here are 3 Spuffy/BtVS fics:-
1) Replacement by Sus (beware N/C 17)
Summary: After years of struggling for a life of her own, the immortal Slayer is finally free from the demands of her friends and the Council. Will she be willing to go back in time and do it all over again, just for the chance to be with the vampire that was always meant to be her other half? An enemy from the future will give her the chance, if she wants it.
2) It Came Upon A Midnight Clear by Holly (beware N/C 17)
Summary : Buffy and "A Christmas Carol"
3) Echoes by Holly (beware N/C 17)
Summary : A slayer barters with a demon to rescue her lover, and finds herself unwittingly projected nearly three hundred years into the future with no memory of the life she left behind.
That took ages to find 3 favourites .... out of hundreds!!