4. The Death, Resurrection, And Death of Bart Bass
This one spans the whole series. In the middle of the second season, Lily receives a call saying that Bart has been in an accident. The episode ends and the audience isn't treated to anymore information. Once Gossip Girl came back after a midseason hiatus, it was time for Bart's funeral. No real explanation of the car crash or how it came to happen. Seem fishy? Definitely, but the writer's know that you'll forget about it for three more years until they're ready to bring Bart back in Season 5. Surprise! Just when you think everyone is finally doing well and the series could probably end, we find out that Bart never really died and has come back to swoop in and take Bass Industries from Chuck, thus launching the plot for the final 10 episodes that made up Season 6. To sum it up, three years of faking his death haven't softened Bart one bit. After an exhaustive search for proof of Bart's alleged felonies, the penultimate episode of the series climaxes in a father/son brawl which leaves Bart hanging from a rooftop set to a hilariously campy score. Chuck stands back, Bart falls to his death, and the series can finally end.
3. The Beginning of Blair and Chuck
Chair, one of the greatest (and most-obsessed over) television couples of the early 21st Century, began suddenly and shockingly. Chuck and Blair had always had chemistry, but up until Episode 7, everyone thought Nate and Blair would eventually work it out. It was only when the characters themselves realized that they wouldn't be together that Blair slipped inside Chuck's limo and a famous couple was born.