They're all impressive in their own ways. "Chosen" is cool for the sequence of Buffy and Willow empowering all the potential Slayers at once, changing the rules of the game; it's a very inspiring sequence and a cool resolution to the series.
"Not Fade Away" isn't on the top of my list, since it's more tragic and more ambiguous, and feels a bit rushed to me. Still, it's a nice reaffirmation of the heroes' principles, and certainly ambitious and daring. Kind of undermined by the comics continuation, though, I felt.
Serenity is an excellent movie and a satisfying resolution to River's arc. It's hard for me to draw a direct comparison since it's a movie rather than a TV series finale, making it different in scale, pacing, etc.
I really liked the resolution of Dollhouse in "Epitaph Two." I think it has a very impressive and intriguing resolution; I love it that Echo doesn't actually save the day. Rather, her role was to bring together the team, to be the inspiration for these formerly amoral people to become the heroes that would ultimately save the world. It's a nice, err, echo of Eliza Dushku's approach as the producer of the show -- she was always admirably willing to step back and let the ensemble shine rather than making the show into a vanity vehicle for herself. I also think it's nice that for once a featured romance in a Whedon series actually has a happy ending, in the way that matters. Though I figure the only reason that happened was because he knew he wouldn't be revisiting the show.
