4) Welcome To Earth-2/Escape From Earth-2 (Season 2, Episodes 13-14)
After delivering time travel in season 1, season 2 went even further in having Barry and friends discover the existence of parallel worlds. In this two-parter, we finally got to see the gang travel to the slightly different yet eerily similar Earth-2 (with its throwback fashions and tech) and, as is the time honoured tradition, meet some alternate versions of themselves.
Most of the episode’s fun comes from the roll call of these doppelgangers – Joe as a jazz singer, Iris as a detective, Caitlin as Killer Frost, Cisco as Reverb, even Arrow’s Deadshot as a comic bumbler. Of course, there’s also some dramatic stuff too, as the team lead a daring trip in Zoom’s lair to rescue Jesse.
These episodes also represent a massive turning point in the season as a whole, with Jay ‘dying’ at the climax of episode 14 and being revealed to be Zoom in the very next episode.