But which Joss series is the most re-watchable? Well, here's my personal opinion, going in reverse order (oh, and I'm leaving "Dr. Horrible" out of the ranking for two reasons: 1- it was a web series, not a network series and 2- it's only 42 minutes, which means its about the same length as a single episode of the other series, giving it an unfair advantage in re-watchability).
4. "Dollhouse" (27 episodes, including unaired pilot)
Well, one of them had to be last, and unfortunately it's the most recent series. I actually love "Dollhouse", but it's at its most re-watchable when you omit entire episodes. I recommend starting with the unaired pilot, then watch episodes 6-12 of Season 1. Skip ahead to episode 2 of Season 2, then watch through episode 12. Then watch "Epitaph One", followed by "Epitaph Two: Return". That actually comes out to 21 episodes, which is one shy of a standard TV season.
3. "Angel" (110 episodes)
Why is "Angel" only #3? Well, let's answer that question in song:
You came and you killed off our Cordy
You led us astray
Your entire storyline bored me
So please just go away, Oh Jasmine
Not all of Season 4 was terrible, but between the Jasmine stuff and the W&H stand-alone episodes early in Season 5, the show hits a huge lull in re-watchability.
2. "Buffy the Vampire Slayer" (145 episodes, including unaired pilot)
I actually feel bad only ranking Buffy as #2, but if you're a long-time reader of this blog, you may remember Project 310 from last year, when I re-watched the first four seasons of "Buffy", one episode a day, leading up to March 10, when I watched all of Season 5 in one day. I definitely had times during the project when the whole thing dragged, particularly in Season 4 (I'm staunchly anti-Riley). Plus, at 144 (or 145, if you count the original half-hour pilot) episodes, it's just not as nimble as my #1 choice...
1. "Firefly" (14 episodes, including unaired episodes and counting the original pilot "Serenity" as one episode)
There aren't many silver linings to "Firefly" having been canceled so soon. One of them (perhaps the only one) is that at 14 episodes, it's easy to pop in your "Firefly" Blu-ray discs and blow through the series in a weekend (or, as I've done, in a lazy Saturday). When you add in the "Serenity" movie (and yes, I know I'm cheating now), you get a nice conclusion on top of the series itself. Bookending the series with "Serenity" the pilot episode and "Serenity" the movie, you get a great beginning-middle-end narrative that you can watch from start to finish multiple times a year.