Man, some tough choices here (well, except for "Earshot," which I don't rate all that highly.)
I gotta say, though, it's a little problematical for me in more ways than one.
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Nightmares: Very good episode, but if I had to pick a single episode from Season 1 as a standout, it has to be "Prophecy Girl". So that takes "Nightmares" out of the running for me as Series Standout.
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Halloween: I admit it: this is one of my all-time favorite episodes, easily in the top five or six. But I make a distinction between "Favorites" and what I call "Best Of", even though there's considerable overlap. And, for me, the standout episode of Season Two is "Becoming, pt 2", so, unfortunately, that takes "Halloween" out of the running as Series Standout.
But... Rocker Ghost Willow? :jaw_drop:
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Earshot: M'eh. Never really cared much for it. The Season 3 standout for me is "Anne", although my favorite has to be "Doppelgängland". So, "Earshot" isn't even close for me in this poll.
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Hush: Here's where it gets tough. "Hush" is easily around the top of both my "Favorites" and my "Best of" list, both for the season and the series. And it certainly takes my vote as the standout for Season 4. Plus, it has Willow. And Tara. Absolutely a Finalist.
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The Body: It will never make my "Favorites", but it's certainly on my "Best Of". Give an award for "Most Traumatic," and it would be a contender for that one, too. Finalist #2.
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Once More, With Feeling: Another one that's up around the top of both my lists. The standout of Season 6, and Finalist #3
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Conversations With Dead People: Hm. <sigh> You know, I want to like this one; it's well written, (mostly) well acted, and creepy as all hell (though not as creepy as "Hush," by a long shot with a big gun.) And, while I would have liked to see Amber return, that's just a selfish want; I personally think that, within the context of Willow's already overbearing grief, it works better the way it played out, with the thought planted in her head that it's her fault that Tara can't appear to her being just one more brick in the load she's already carrying - and about to break under. But still, I just can't get this into either my "Favorites" or "Best Of" categories. There are several in Season 7 I like better, and this one just doesn't make the cut.
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So that leaves us with three finalists.
"The Body" is downright hard to watch. The sense of drifting, helpless, numb, is almost palpable, and anyone who's had to deal with the death of someone close to them knows how it feels. This will affect Buffy's actions and her mental state, for the foreseeable future. But for all that, there's a great deal about it that also could be had on any other show (except, of course, for the lack of little things such as writing, directing, and acting on other shows,) so I can't really give this the standout of the series. So, third place for "The Body." Leaving:
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"Hush" or "Once More With Feeling?"
Well, "Hush" has just about everything, starting with an experimental format. And it works. The images, the soundtrack, the touches of black humor, everything clicks. The Gentlemen are pretty much the creepiest villains of the entire series; who hasn't had nightmares with the inability to scream? Buffy and Willow holding hands as they walk through a silent, miserable town; Giles and his slide show with its appropriately macabre musical accompaniment; Buffy's crashing entrance through the boarded-up window; Tara, so shy she can't even speak up in the Wiccan group, but strong-willed enough to risk going out alone at night to try and help; and the surge of power when she and Willow clasp hands. This episode is the beginning of some huge changes in the Scoobies' lives - but it's only the beginning, and that's the reason I have to give the "Standout of the Series" award to:
"Once More With Feeling." Which also takes an experimental format and runs with it. I'd bet real money that anyone but Joss would have dubbed the actors' voices (well, with the possible exception of Amber. Although, given the way Hollywood does things, they'd probably all have been dubbed. With people who not only didn't sound like the actors, but likely wouldn't have done any better job singing.) It's also unusual for a musical to actually have the singing and dancing make sense within the internal context of the show, consistent with the plot. But the real reason I give this one the Standout award is: this is where it starts to fall apart. This is the payoff, when the gang's bad choices begin to catch up with them, when all their secrets start coming out. Is this a Good Thing? Hell, no. It's the start of some remarkably Bad Things for our friends. But from a dramatic standpoint, you betcha. This is when they begin to learn that nothing is free, everything comes with a price; you can lie to your friends, you can lie to yourself, but eventually it all comes crashing down. It's the start of a long slide down for the entire gang. At the end, some will come back stronger (for a while.) Some won't come back at all. And they'll all be changed forever.