'NCIS' 400th Episode: Where Does It Rank Against the Previous Milestones?

Jace Downs/CBS; Sonja Flemming/CBS; Richard Cartwright/CBS
NCIS has celebrated another milestone in its 18th season, and it's quite the feat: its 400th episode.
The hour revealed how Gibbs (Mark Harmon) and Ducky (David McCallum) met, and fans were treated to returns from Mark's son Sean Harmon and Adam Campbell to play their younger selves in the flashbacks.

10 Heartbreaking & Powerful 'NCIS' Episodes That Left an Impression
We take a look back, from the heartbreaking (Kate's death) to the powerful (Taye Diggs' guest spot).
In honor of that milestone, we're taking a look at all the ones the procedural has marked so far. They've brought a team member on officially, introduced an agent's relative after years of only hearing about him, featured memorable cases, and been among the best episodes of the series. Scroll down to see our ranking, from least to most memorable.
NCIS, Tuesdays, 8/7c, CBS