The best I have read so far was
link, it's pretty long, but it's probably the only fanfic that manages to keep Severus Snape in character from the beginning till the end and at the same time make him quite a romantic man. Hermione might be a bit OOC, but I kinda like her that way :)
The other good fic is
link, but this one is quite heavy with smut and BDSM, so if আপনি don't like the theme, I suggest not reading.
There was also another one, called Invitation to Detention, but it's been pulled of from Ashwinder and now I can't find it anywhere :(
Besides that there's a lot of great fics on Ashwinder and FF.net, but way too many to তালিকা them al. I just listed those that made the biggest impression on me.
edit:
this is the third one I am re-reading this one and I প্রণয় it so I thought I'll mention it:
link. Great story (even though I always skip the Snape/Lily thing)