Saw the poll in the fiction section and decided to start a discussion here since nobody else had
I voted for Laurence from The Adversary. This character had a huge impact on me, (and this is one of my all time fav stories ever - I think Laurence was a large part of that).
He is so well written, so fully developed, clearly evil and yet has some frailties too, which only made him so much more real. He is a much more complex man than first appearances would have you believe, and that added such incredible depth and emotion to that story, which was as much about him as it was Mulder and Skinner, and sometimes i found myself trying to think if he had actually been mentioned in the show, he was just so perfectly written into that world that he didn't seem at all like an OC.
In many fanfics, i find myself resenting the OC as they are often just seeming to be there for the sake of being there, and since we know the other characters so well from the series, or other fanfics, there is this strange character that seems woefully under-developed and kind of pointless.
Laurence though, and his lavender, years after reading that story, have stuck with me and my mind has rewritten it as him actually being a part of the X-Files series, hell he is more X-Files than Doggett ever will be to me.
As far as best characters of all time, he is up there with McKay, Londo... and Laurence.