My list:

Buffy: Within the show's main timeline, I'd say after she gets the Class Protector award, when Angel walks in at the prom. But she was even happier in the flashback we saw in "Becoming," just before Merrick comes up to her.

Willow: I'd say the beginning and end of S4 were both happy times for her, because of Oz and Tara, and because she was finding a place academically and with magic. She might have been more blissful at the beginning of the season, but she was more self-assured at the end.

Xander: Right after "The Replacement."

Anya: What everyone else said -- rehearsing her wedding vows.

Giles: At the end of Checkpoint. His little girl had grown up, and he'd made his point.

Spike: At the beginning of S2, when he was just gleefully evil. Post-chip, I'd say when Buffy invites him into the house in "The Gift."

Tara: The floaty dance at the end of "Family." She'd found out somethign she believed about herself wasn't true, and learned the people around her really loved her.

Dawn: Hmmm. Maybe the end of "Grave," when she's fought side-by-side with her sister and proven her worth?

Cordelia: When she shows up at the prom in the dress Xander bought for her, I think. It's certainly her most hopeful and dignified moment.

I just wish her last line had some substance, even if her last line wasn't the last line.

Honestly, she talked so much in S7, I was glad we didn't have to hear another Buffy speech at that point. "Spike" said more to me that some 70-word answer about "We all did this, by the power of our friendship and magic and strength and blah blah blah" ever could.

And I liked the smile.

What was Joyce's happiest? End of School Hard?

That's one moment, but I'd put coming home from her date in "I Was Made to Love You" above it. She was in a place to really appreciate small things -- health, having a date, bantering with her daughters.

but I'd put coming home from her date in "I Was Made to Love You" above it

This certainly fits the ME profile of having people be their happiest right before something awful happens to them - see also Passion, Surprise/Innocence, etc.

Buffy: How about jumping off the tower in "The Gift?" She seemed pretty blissful then. Actually, everything between that moment and the end of "Bargaining 1" was probably her happiest time.

Of course, it made everyone else on the show miserable...

Dawn: Beginning of season 7. Starting high school, finally has a good relationship with her sister, accepted as (more or less) an equal in the Scoobies...actually, all of "Lessons" up until Buffy bursts into the classroom to "save" her.

Make your own William Doll

It seems to me that it could also be a Randy doll with a haircut and some product.

How about jumping off the tower in "The Gift?" She seemed pretty blissful then. Actually, everything between that moment and the end of "Bargaining 1" was probably her happiest time.

Y'know, I just can't count fulfillment of a death wish as "happy."

Maybe it's just me.

I never saw that as fulfillment of a death wish. I think I am very alone. I never saw it as suicidal. I never saw it as giving up. I just saw it as her next turn at throwing her life between death and people she loves, and this time, she knew she'd really save the world.

and this time, she knew she'd really save the world.

Again. Also, if she didn't, if she was wrong, she wouldn't be around to care. And if she's wrong, the world ends and so does Dawn. One of the many things that make season 5 the bottom of my list. They could have at least forshadowed the damned "our blood is the same thing" somewhere/anywhere in the season. It didn't have to be obvious - just something to point back at (if they'd taken as much care with this as for the stupid-ass "Giles not touching anything"...rant, rant, rant). People complain about all the little things in season 7 that could have been done to make it a little more coherent, but season 5 remains to me the season of wasted potential (as opposed to wasted potentials) where a few tweaks could have made it brilliant. Leaving aside Charlie Weber, for whom there was no hope in the acting sweepstakes, I thought Claire Kramer had her moments, and, with some judicious care, they could have made her at least as good as the Master.

I have an alternate take on the whole bloody season 5 rambling around in my brain - maybe it's time to work that out, finally.

Y'know, I just can't count fulfillment of a death wish as "happy."

Maybe it's just me.

I never saw that as fulfillment of a death wish. I think I am very alone. I never saw it as suicidal. I never saw it as giving up. I just saw it as her next turn at throwing her life between death and people she loves, and this time, she knew she'd really save the world.

I see it as a moment of perfect clarity, where she finally realizes what she has to do. She knows how to save the world, her friends, and Dawn, and how to make everything all better in one fell swoop. Coming at the end of a period of intense bleakness and confusion for her, making this leap must have seemed like the most perfect thing in the world. Of course, the fact that doing so would stop her own pain, let her rest, and absolve her of all future responsibility probably didn't hurt. There's a fair amount of selfishness involved in this act, not to mention the hypocrisy of her telling Dawn that the hardest thing in this world is to live in it--then making a beeline for the exit.

But I don't think Buffy was thinking of the negatives, or of the pain she would cause her friends. She saw the jump as Noble Sacrifice+No More Slayer Crap+Fulfilling the "Death Is Your Gift" Prophecy=Happy.

They could have at least forshadowed the damned "our blood is the same thing" somewhere/anywhere in the season. It didn't have to be obvious - just something to point back at (if they'd taken as much care with this as for the stupid-ass "Giles not touching anything"...rant, rant, rant).

They did at least two times that spring immediately to mind - in Blood Ties with the 'Summers' blood' comment and again in Forever, when Doc yanks out one of Dawn's hairs, examines it, and pronounces that she has very strong DNA.