One if the best of all-time. I'm currently rewatching. I was in college at the time so I kind of missed the last couple of seasons. It's much better than Fringe IMO.
X-Files - Recommend or Pass
X-Files is my favorite show of all time. I love many of the characters (not just Scully and Mulder, but also Skinner, CSM, Krycek, X, etc).
Season 1 is good, a bit clunky at times.
Season 2 is excellent
Seasons 3 through 5 are the apex
Season 6 is excellent
Season 7 is hit or miss
Season 8 and 9 are decent, but they pale beside the rest of the show.
I liked the mythology episodes more the first time through (esp for the first five seasons or so), but I think many of the standalones (Monster of the Week) eps have higher re-watchability. Also, as has been well documented, the mythology kind of went of the rails at some point.