I think that season five could've been great. Before anyone starts throwing rotten vegetables, let me explain!
If it wasn't EFC but another series entirely, I would've been interested. Howlyn, Juda, Zo'or-with-breasts, they were hot. And, the idea of an alien race being ressurected only to be the dominant sentient life on Earth before us is an interesting story. But, I loved EFC not because of hot life sucking vampire aliens bent on screwing, eating or turning humans, but because of oddly attractive and other worldly aliens that had these complex motivations and intensely complex personalities and relationships.
What I enjoy and appreciate about EFC is humanity and the Taelons, not season 5 and not really the Jaridians or stereotypical human-alien heroes. (that's why my rpg and fan fiction is season one.)
But as far as the best season 5 episode goes, I'd say Boone's Assassin. Boone was a great character that never should've been 'retired' and you get to see Zo'or being...awesome. Finally the outside and biological abilities seem to reflect the person within!