This is kind of tricky. I would recommend very strongly against following silenus’s list. They’re all excellent episodes, but without knowing the context for the events and characters, they’re either not going to make much sense, or they’ll lose a lot of their emotional impact. The Body is one of the best hours of television ever filmed, but if you haven’t spent five years getting to know these characters before hand, it’s not going to be the emotional suckerpunch it was meant to be. Plus, if you get hooked and want to watch the whole series after all, it’ll spoil a lot of the surprises that made the show so awesome.
Probably your best bet, if you’re renting and just want to test the waters, is to get Disc 3 of the first season. It’s the final disc for that season; Buffy was a mid-season replacement, so the first season only ran to twelve episodes. And a lot of that time was spent with the show still trying to find its legs. However, by the end of the season, it had started to really hit its stride, and the last four episodes are all really good. However, it’s still early enough in the series’ run that there won’t be a lot of continuity you need to follow, and the spoilers won’t be too bad. Disc three has a good cross section of the different genres Joss Whedon worked with in the series: comedy (The Puppet Show), horror (Nightmares), allegory (Out of Sight, Out of Mind, although this ep is pretty heavy-handed about it), and high drama (Prophecy Girl). It’s not the best of the best, but it’s good enough to give you a feel for the show, without ruining too much of it for you.
To help you out, this spoiler box contains brief bios of the principle characters in the show to date:
[spoiler]Buffy Summers: Vampire Slayer by night, “that wierd girl at school” by day.
Giles: School librarian and member of the Watcher’s Council, an ancient order dedicated to finding and training Slayers.
Xander Harris: School loser and Buffy’s friend. Has a huge, unrequited crush on Buffy.
Willow Rosenberg: Another school loser, and also Buffy’s friend. Has a huge, unrequited crush on Xander.
Cordelia Chase: Most popular girl in school, and Buffy’s archenemy.
The Master: Ancient vampire magically imprisoned beneath Sunnydale, and Buffy’s other archenemy.
Angel: Used to be one of the most evil vampires in the world, 'til gypsies cursed him with a conscience. Now fights for the good guys. He and Buffy totally have the hots for each other, but they’ve got “issues” keeping them apart.
Mrs. Summers: Buffy’s mom, who has no idea that her daughter is a superhero.[/spoiler]