I found this site averaging out ratings of BTVS episodes. It's several years old, and I also looked up some new 20th-anniversary best-episodes lists, which showed some strong consensus. (Google "best buffy episodes," you'll find plenty.)
It occurred to me that, rather than make my own tiresomely repetitious best episode list (the major variant is that I'm less fond of the season-ending Big Battle episodes than most people are), I should offer my opinion of the most under-rated episodes. I mean, we all know that "Innocence" and "The Wish" and "The Zeppo" and "Doppelgängland" and "Something Blue" and "Hush" and "Who Are You?" and "Restless" and "Once More with Feeling" and "Conversations with Dead People" and the like are great, right? But what about the ones that don't make all the best-of lists or get high ratings in complete evaluations?
DB's Most Under-rated Buffy the Vampire Slayer Episodes (chronological order)
Lie to Me (2.07)
During the show's heyday, I was on a lot of convention panels about it. On one, we were posed the interesting question: since this show took a while to hit its stride, what's the best episode to introduce people to it with? Had to be a stand-alone, had to hit the major themes, had to be good. Ben Yalow suggested "Lie to Me," the vampire wanna-be episode, and I think he was right.
What's My Line, part 1 (2.09)
Worth it for the (at the time) stunning final line: "I am Kendra, the Vampire Slayer."
Faith, Hope and Trick (3.03)
The first episode I saw. Evaluations mostly diss Mr. Trick. I thought he was a great villain, so suave and cool.
Gwendolyn Post! Another great guest spot, and an interesting preview variant on Wesley. I was so sorry when Joss didn't hire the same actress to play Adelle on Dollhouse.
The "witch-hunt" episode is important as the only one to face head-on the peculiar premise of mid-period Buffy, when the vampires had become too common to support the early seasons' "secret history" premise, but were not yet openly acknowledged. People were just in denial of the obvious, and this one shows that in operation.
The "Mr. Light Show" episode. Awesome drama with sinister implications.
The Freshman (4.01)
Had a particularly good gang of guest vampires. "Are we going to fight, or just have a giant sarcasm rally?"
This Year's Girl (4.15)
The essential prelude to the immortally-good "Who Are You?" Faith's blistering encounter with Buffy and Willow is one of the most dramatically intense scenes of the entire series.
The Jonathan episode. No further comment should be necessary.
Real Me (5.02)
A lot of viewers dislike Dawn, who's the central figure here. I don't; I find her funny rather than annoying. But even better, this is the episode in which the airhead vampire Harmony had her gang of minions, one of whom was played by Tom Lenk, who later returned as Andrew.
The yet-to-be-famous Amy Adams plays Tara's Cousin Beth. She only has one big scene, but it's a stunner.
Introduction of the Buffybot. Her computer readouts are a delight, and so is her Anya-like conversation, especially with Anya. Another demonstration of SMG's acting versatility.
The Weight of the World (5.21)
Widely disliked, but I found this episode in which Willow rummages around inside Buffy's comatose subconscious to have some of the same surreal quality that made "Restless" so good.
And that's about where my real good memories of lesser-known episodes runs out, sorry.