Season 8, Episode 13: "If/Then"
Is an alternative-universe plot a little gimmicky? Sure, but this episode is so much fun. It imagines a world where Meredith is a preppy It girl engaged to a wildly energetic Alex, her mother alive and happily married to Chief Webber. Derek no longer has his easygoing disposition — he's a sad sack married to Addison. And Cristina — where do we even begin? She's a goth queen with straight hair and blunt bangs. But even with all of these weird switches, all's well that ends well. The characters somehow find themselves exactly where they should be — like Meredith and Derek chatting each other up at a bar.