Ok, well, since CappucinoGirl has set the tone by posting her faves, i'll do the same.
Going way back, this one of the first fics I read, and definitely the one that sparked my interest in CSI fanfic at all.
All in War with Time by AllthingsMSR and SorchaMise As a general rule i'm skeptical of fics where something happens to Sara, but this one is written with such obvious love for and knowledge of the characters that I like it because of what happens, rather than in spite of it. The author never gets out of character and has a real flair for humor and angst; something I aspire to.
The Better Claim by EOlivet You want angst? I mean do you really, truly want angst? The kind of angst that just tears you up inside without ever resorting to cheap tactics or cliches? This is your fic: a truly great fic amongst great fics. Just don't say I didn't warn you.
Stealing Signs by EOlivet I can't say enough about this piece. If you're a stickler for the plot or situation being something that would absolutely happen on the show, why are you reading fanfic? Seriously, though, the interaction between the characters is chock-full of dead-on vibey UST.
Here's where this gets ugly. Midnight Caller is one of the best, along with one of the most prolific Grissom/Sara authors around. Her PG is as good as her NC-17 which makes it so difficult to choose one of her stories as my favorite. So I'm going to pick three. *ducks*
Truth Serum - There's nothing funnier.
Splendor in the Grass - Except maybe this one.
Solarization - Gorgeous descriptions. Tons of UST.