The DOP on the show was a guy called Bill Roe. His work on the X-Files won him the ASC Award for Outstanding Achievement in Cinematography in a TV Series (he was also a nominee a number of times). I would personally say that at times the show reached a level of cinematography on a par with cinematic quality (and at times better). The look of the X-Files has been shamelessy copied ever since to the point where people don't know what they are looking at is a rip-off because it's so common (kinda like the Matrix / bullet time).