It is without a doubt 'Paralyzed'
Summary: When Buffy doesn't die at the end of season 5. In the summer between 5 and 6, Buffy and Spike begin to explore their relationship. Buffy is struck down by demon poison, and must reply completely on Spike for everything. Of course the Scoobies aren't too happy with that.
You should probably like 'Walking After Midnight' by Chelle too.
Summary: When Buffy is paralyzed during a routine patrol, her whole life falls apart. Making matters worse is the fact that Joyce can't afford to hire the full time care that Buffy now needs. Enter Spike, who is recently chipped and being targeted by the same group of demons who harmed Buffy. Joyce views it as an answer to her prayers when Spike accepts her offer of caring for Buffy in exchange for a safe place to live. Buffy, on the other hand, takes a little lot of convincing. This was written during Season Four when it was airing. I hope you enjoy this cleaned-up and rewritten version of WAM.
The first version here
and the remastered here