Flukeman!!!!! The look of flukeman itself is enough to make the episode a winner. Part worm, part human/mutant/whatever, flukeman is a sight to behold. It's a worm in that it's white and has weird teeth, but it has a distinct head and body. Could your local sewer contain one of these? Probably not, unless you live in a parallel universe where radiation from Chernobyl can create perfect worm-human hybrids.
And it's a good thing flukeman makes you squirm. The plot of "The Host" has no meat on the bones, and when dealing solely with a creature and not humans, there really is nowhere the plot can go.
Now that the X-files is gone, the paranormal continues to fall right into Mulder's lap and Scully is along for the ride as well. Mulder gets a mysterious call from someone who claims to be inside the FBI and helping him. He says cheesy lines "Reinstatement of the X-files most be undeniable," but hey, any help is good.
Looking at the plot, "The Host" would probably rank very low. However, the flukeman, which everyone remembers, makes the episode one of the most memorable episodes of the series and that's good enough for me.
Score: 9.0/10