I've written up some lopsided lists before, but this is one of the most extreme. Eight of the ten picks below are from the first two seasons, which I found significantly better than the latter three. However, it's a mistake to write those latter seasons off completely, as several major characters only came into their own late in the series run.
Picks below are unranked and ordered by airdate. Moderate spoilers for the first two series ahead:
"Natural Selection" - It's so much fun to watch Sarah flying by the seat of her pants, trying to step into Beth's shoes after stealing her identity. Confronted with one crisis after another, the audience learns new information along with Sarah, and it's fun trying to puzzle out what's going on. Looking back, it's also interesting to see what a different kind of show "Orphan Black" was at the outset, far more grounded and serious about its central mysteries.
"Effects of External Conditions" - Our introduction to Helena, who was immediately one of my favorite characters. Her impersonation of Beth, and Alison's impersonation of Sarah, also cemented for me that Tatiana Maslany's performances were orders of magnitude above anything I'd ever seen in a genre program. The clones-playing-other-clones trick was something that the creators wisely limited to only a few times a season, and it was never as effective as the first time they did it.
"Variations Under Domestication" - Alison's first big episode sees her trying to figure out whether Donnie is her monitor, while also hosting a party for her neighbors . Alison's comedic turns were always highlights for me, and the show got a lot of mileage out of poking fun at her suburban life. Here, turning golf clubs and glue guns against her husband tells us so much about how Alison operates. Also note that this is the first time that Vic has an unpleasant altercation in the Hendrix garage.
"Endless Forms Most Beautiful" - Honestly, this episode is a little pat in the way that it ties everything up. However, it introduces Rachel, who would quickly become another of my favorites, and it starkly underlines one of the major themes of the show by revealing that the clones are considered intellectual property by their creators. In a series that manages some really creepy moments, this is one of the best. Also, I did not see the Aynsley storyline playing out the way it did.
"Governed as It Were by Chance" - Helena certainly knows how to make a good entrance. After Sarah's latest investigation into Rachel's background goes sideways, her storyline and Helena's converge in the most violent way possible. I love that at this point, after several weeks of seeing Helena as the victimized prisoner of Henrik and his flock, the show can still use her as this terrifying force of horror and mayhem at the drop of a hat. Maslany's reactions to her as Sarah really sell it.
"Knowledge of Causes, and Secret Motion of Things" - Another darkly funny Alison episode, centered around Sarah's visit to Alison and Vic's rehab center on Family Day. However, it's Donnie who turns out to be the real star here, when he finally learns the truth about the monitoring program and decides to confront Dr. Leekie. This leads up to another shocker of an ending that I didn't see coming. And after this point, Donnie would officially become the Clone Club's best comic relief.
"Things Which Have Never Yet Been Done" - The episode where Donnie and Alison have to bury a body in their garage and fend off a nosy Vic. This is one of my absolute favorites, because it's when the Hendrixes really become a team, both in crime and in comedy. And in subsequent seasons, when things started going off the rails, they remained dependably fun to watch. This episode also pulls off a clone impersonation that I didn't see coming, which sets up the end of the second series.
"By Means Which Have Never Yet Been Tried" - There are a lot of big reveals in the second finale, but I like the episode best as a Rachel episode. After toying with being sympathetic all year long, here she suffers a terrible loss and we see her really commit to her villainy. Cosima also gets one of her rare early spotlights where she gets to do more than just be a source of exposition or further a romantic subplot. And one of my biggest disappointments is that we never got more Marion after this.
"Ruthless in Purpose, and Insidious in Method" - Krystal, despite being introduced so late in the show, was a great clone character. Here, she's being investigated by an undercover Felix. This allows Jordan Gavaris to use a different accent and Tatiana Maslany to embrace her inner bimbo, and it's a delight. It's also a pretty good Rachel episode, where there's a major development in the ongoing subplot about her recovery. I think furious, in-recovery Rachel may be my favorite Rachel.
"To Right the Wrongs of Many" - The last series of the show was only middling, but they found a great way to end it. Sarah is in danger of winding up right where she started, except that she's gently reminded that now she has a support network of loving sisters to lend an ear. The show's special effects wizardry, allowing for the multiple clone sequences, was never utilized better than in the big family bonding scene. And who Helena names the babies after, tickles me to no end.