Key EpisodesRodney appeared in all 100 epiodes of Stargate: Atlantis, as well as 6 episodes of Stargate: SG-1 and while they're all worth a watch, there are some episodes are more important than others when it comes to the good Doctor McKay.
SG-1 5x14 - 48 HoursRodney's first appearance. Establishes his somewhat abrasive personality as well as his relationship with Carter and the reason he was sent to Russia.
SGA 1x01 & 1x02 - Rising, Parts I & IIRodney joins the expedition to Atlantis and plays a key part in figuring out how the Ancient technology there works.
SGA 1x03 - Hide and SeekRodney receives the ATA gene therapy, allowing him to activate various pieces of Ancient tecnology. He also sows the seeds for his friendship with John Sheppard and joins Sheppard's team.
SGA 1x09 - HomeRodney seemingly returns to Earth and we get a look at his old apartment and life (or lack thereof).
SGA 1x17 - Letters from PegasusRodney records a message to be sent back to Earth in a database and reveals that he wishes he and his sister were closer.
SGA 2x04 - DuetRodney is forced to share his body with Lieutenant Cadman's consciousness and goes on his first date with Katie Brown.
SGA 2x06 - TrinityRodney believes he has solved a problem the Ancients had with an experimental power generator and ends up destroying a solar system, losing Sheppard's trust in the process.
SGA 2x14 - Grace Under PressureRodney is trapped in a Puddle Jumper at the bottom of the Lantean ocean and begins to hallucinate Carter due to a headwound as his team race to save him.
SGA 3x08 - McKay and Mrs. MillerRodney visits his sister and convinces her to return to Atlantis with him after she comes up with a theoretical proof which can be applied to his work.
SGA 3x14 - Tao of RodneyRodney's intelligence begins to grow by leaps and bounds after his DNA is altered by an Ancient device which threatens to kill him unless he can ascend.
SGA 3x15 - The GameRodney discovers that the 'game' he and Sheppard have been playing has actually been relaying instructions to real populations and that his fictional Kingdom of Geldar really exists.
SGA 3x17 - SundayRodney says an emotional farewell to Carson Beckett and returns to Earth to act as one of the pallbearers at Beckett's funeral.
SGA 4x02 - LifelineRodney successfully reactivates the code which instructs the Replicators to fight the Wraith but at the cost of leaving Dr. Weir behind.
SGA 4x04 - DoppelgangerRodney becomes infected with a crystalline entity which has been making it's way through the Atlantis personnel and Sheppard takes a trip into Rodney's subconscious.
SGA 4x09 - Miller's CrossingRodney travels back to Earth after his sister is kidnapped, only to find himself captured by the same people who took her.
SGA 4x13 - QuarantineRodney's plans to propose to Katie Brown are interrupted when Atlantis goes into quarantine mode, making him realise that he's not ready for marriage.
SGA 4x16 - TrioRodney becomes trapped in a mine with Carter and Keller and emerges from the situation with a date with Keller.
SGA 4x20 - The Last ManRodney spends 25 years working on a way to alter the timeline when Sheppard goes missing and successfully sets up a hologram of himself to aid Sheppard 28,000 years in the future.
SGA 5x01 - Search and RescueRodney delivers Teyla's baby.
SGA 5x06 - The ShrineRodney contracts Second Childhood and begins to lose his memories, prompting him to keep a video diary of the progression of the disease. During the filming of this diary, he tells Keller that he loves her.
SGA 5x09 - TrackerRodney works with Ronon to rescue a kidnapped Keller and the two men agree to let the best man win when it comes to her heart.
SGA 5x10 & 5x11 - First Contact & The Lost TribeRodney works with Daniel Jackson to uncover Janus' secret lab. When they are kidnapped by the Asgard, they work together to escape.
SGA 5x16 - Brain StormRodney, accompanied by Keller, returns to Earth to attend a physics presentation which he discovers is based on work stolen from him.