I see a lot of my favorites already posted here, but there are a few still worth mentioning.

This sold me on J2 high school romance:
“So Few Words” by anyothergirl415
http://masterlist415.livejournal.com/38437.html#cutid1

Here is where I fell in love with crackfic:
“The Incestuous Courtship Of The Antichrist’s Bride” by fleshflutter
http://fleshflutter.livejournal.com/102268.html

This made me a fan of pre-series Dean doing whatever it takes to take care of Sammy:
“God Made Boston On A Wet Sunday” by balefully
http://balefully.livejournal.com/617895.html#cutid1

This gave me epic, crazy casefic love:
“Not Time’s Fool” by pandarus
http://pandarus.livejournal.com/257110.html