In the first season, then-Senator David Palmer angrily confronts Jack Bauer who he's been wrongly led to believe has been trying to kill him. Jack furiously explains that both he and his family have nearly died because Jack has been working to save him. The look of shock, embarassment, and understanding on Palmer's face are priceless as he realizes that Bauer is a patriot who has saved him and his family. It is the start of a friendship that would be central to the show for the next several seasons.
David Palmer resists the temptations of one his staffers and proves (somewhat sadly) that no real President will ever have that kind of character.
Watching the Bauer family reunited (if only for a moment), the relief they on Jack, Terri and Kim's faces every time the hugged was worth the wait.
When Teri finally tells Jack she's pregnant again, Jack looks like he could jump over the moon with joy.
George Mason's speech to Michelle in Season 2.
Mason: Believe it or not, I used to want to be a teacher. A long time ago. You know why I didn't? Because DoD offered me more money. That's how I made my decision. So I made myself miserable, and I made everybody around me miserable. For an extra five thousand dollars a year. That was my price. ...You know, Michelle, I'm not a big advice giver, but under the circumstances, don't wait around for your life to happen to you. Find something that makes you happy. Do it. Because everything else is just background noise.
Day 7: 10:00pm-11:00pm introduces us to Port Authority supervisor Carl Gadsen, a Nice Guy who's wife is about to have a baby. Turns out he was promised some cash for looking away the one time he shouldn't. Jack and Tony capture him and explain the situation. He feels terrible, and though frightened offers to help with Jack's promise that he won't let him get hurt. Turns out the only way Jack and Tony's newest scheme to catch Stokes men will work is if they let him get killed to maintain their cover. Just when it looks like Cooper is going to kill him, Cooper's shot by Jack, who couldn't abandon the poor man.
The end of Season 7, in the White House, when an exhausted Allison Taylor allows herself a moment of weakness, confessing to her former Chief of Staff (and close friend) Ethan Kanin, "I've lost everyone." Ethan says simply, "Not everyone," and she correctly concludes that he is un-resigning. The look of dawning hope on her face is simply magnificent.
George Mason patching things up with his estranged son, John.
Mason's speech to Jack that convinces him to drop his Death Seeker attitude and find a reason for living again as well as Jack's reunion with Kim at the end of Day 2.
Jack's goodbye to David Palmer at the end of Day 4 (which also makes it a bit of a Tear Jerker when you remember it's the last time they'll ever communicate on the show)
In Season 6, Martha Logan gets Aaron Pierce to smile.
Jack's last two calls at the end of Day 8.
The last conversation between Chloe and Jack. Chloe's (very likely real) expressions sell the entire scene.
Jack: When you first came to CTU...I never thought it was gonna be you that was gonna cover my back all those years; and I know that everything that you did today was to try and protect me, I know that. Thank you.
Jack and Kim's last conversation, where Kim actually tells him to ignore his Refusal of the Call stance because she understands that if he can somehow make a difference helping to protect Hassan it's worth it.