Ooh, I love the X-files!! So many excellent episodes! My favorites:
"Field Trip"- Season 6, where M & S get trapped in a cave by a fungus, and then they think they're back in the city (but they aren't.)
"Darkness Falls"-Season 1, disappearing logging crews, glowing bugs that cocoon their prey, M & S get cabin fever, very entertaining and interesting.
"The Erlenmyer Flask" - Season 1 finale, discovery of the DNA with and extra pair of nucleotides.
"Ice" - Season 1, another cabin fever episode, with ammonia-based life form in arctic ice.
"War of the Coprophages" - Hiliarious but plausible, ET cockroaches.
"Our town", "Syzygy", etc.