Episode: "Shadow" (Season 5, Episode 8)
Most of Buffy's monsters, be they of the week or season-long "big bads," consisted of guys in rubber suits under extensive make-up. Was it usually painfully obvious? Sure, but the effort put into creating a vast universe of unique creatures was respectable, especially given the sci-fi/supernatural series these days that skirt the issue and just give an actor some funky eye coloring effects and call it a day.
But an A for effort only goes so far, and boy did Buffy fumble at times, especially when it tried its hand at CGI. We'd almost prefer a dude in a costume over this laughably bad snake creature, a rare low moment in the otherwise series-best fifth season.