Season 3, episode 17: "Some Kind of Miracle"
While tending to a ferry accident, Meredith trips and sinks in the water. The attendings frantically do everything to save her, and our beloved heroine enters a surreal dream state where she interacts with ghosts from her past, including Denny Duquette, Dylan the bomb squad guy, her dog Doc, and Bonnie, the girl from the train crash episode. "Some Kind of Miracle" tastefully digs into Meredith's dark, suicidal feelings throughout the third season. She must literally confront her feelings about death as Denny presses her. She envisions her mother telling her that she's anything but ordinary before she comes back to life. So Mer fights. We never expected Meredith to die, given that she's the main character, but that doesn't take away the emotional punch of the episode.