"Day 1, Hour 1. I didn't know what to expect, and [I was] not sure what I was seeing. But I knew I was in for a helluva ride!" — maryb889

"Teri's death still stands as a defining moment for the show because it proved literally anyone was expendable and anything could happen. I had been pulling for a happy ending and it brought me to tears." — CTUTraveler

"Season 2: Nina holding Jack at gunpoint and President Palmer basically giving her a license to kill." — thebarbster24

"Jack saying goodbye (again!) to Kim when he's in the plane he thinks he's going to crash. (And Mason crashed it in his place!)" — Jena H

"Chase letting Jack chop off his hand with the ax so the biological weapon could be put in the airtight freezer before it exploded." — AZTVfan

"By far, Tony coming out of nowhere in Season 4, saving Jack and Audrey, and then taking them to safety." — Annakie

"Definitely President Palmer being shot to death. Yes, Jack's wife dying was awful, but Palmer's death came out of nowhere." — chattypatra

"Day 5, when Edgar Stiles died. It was so sad when he was on one side of the glass and Chloe was on the other (safe) side. They just looked at each other because they knew what was about to happen. And then he got the silent clock. I still get chills!" — ChristiStorm Batts

"First Lady Martha gets the incriminating evidence on her husband, President Logan. And then Logan is arrested by the Secret Service at the end of season 5. Way to go Martha!" — AZTVfan

"What about Bill Buchanan sacrificing himself to save the president because Jack had to live? That's one of the top moments for me." — CastleNBeckett

"Renee and Jack consummate their relationship, only to have a Russian sniper murder Renee moments later." — TVGuide.com editors