So without further ado - fics that left a lasting impression on me (in no specific order):
1) The Xena femslash fic I read when I was 15 (1998) which gave me the first inkling that I might just be gay...
2) Alien: Odyssey (Alien Resurrection - Ripley/Call) - http://www.altfic.com/subtextfic/alienr/prp/alienodyssey1.htm - I read it again 6 or so years later and it is still awesome.
3) Primary by seven_3_oh (Legend of the Seeker - Cara/Kahlan) - http://seven-3-oh.livejournal.com/4232.html
4) Mrs and Mrs Mord'sith by trancer (Legend of the Seeker - Cara/Kahlan) - http://archiveofourown.org/works/182674 mrs and mrs mordsith
5) Trafalgar and Boone Series by Geonn / geonncannon (original fic) - http://archiveofourown.org/series/56493 (I feel like there is an earlier fic that made me a general fan but I can't remember it)
6) There is a Janeway/Seven AU fic where it's a teacher/student relationship that I was utterly addicted to around the time I found BSG (so mid-season 1 of BSG - 2003?) but I can't find it... It was hot...
7) The Girl who would be Queen by starbuck1980 (Once Upon a Time - Emma/Regina) - https://www.fanfiction.net/s/8024564/1/The-Girl-who-would-be-Queen (Yes this is written by my wife but it's amazing. And yes I feel like I should use the LJ link but FF.net came up when I googled)
8) I feel like there is a (projectjulie now) projectcyborg fic that made me a fan of hers but I can't remember it.. but her non-fic fandom stuff on Voyager got me into BSG fandom which changed my life so she gets a place just for making a lasting impression.
9) frogfrizz - again, I can't remember my favorite fic but her imagery is always amazing (also my first beta so honorary fic spot for that).
10) Eye for an Eye by starbuck1980 (BSG - Cain/Gina) - http://starbucks-pen.livejournal.com/39989.html - purely because commenting on it changed my life/introduced me to my wife!
Special mention to pocketwitch - I still remember never really liking Laura/Kara until she wrote them... and Louise (on tamoline's account) whose fic Hot Coffee (Game of Thrones - Sansa/Daenerys) I am utterly addicted to right now -(http://archiveofourown.org/works/912470/chapters/1768424)
Of course if I thought about it a bit more this could be a much longer list... If you care to write your own list link me as I'm curious!