In the episode "Re-do" when Frank rescues Erin and said "Take your hands off my daughter." The villain taunts him and tries to invoke Put Down Your Gun and Step Away ; Frank's response is to shoot him in the head. Made more awesome when you remember that he was using an old .38 Special revolver that was the ancestral weapon of the Reagan family - a weapon older than he is.
In the episode "Smack Attack" when Jamie rushes into the party and gives CPR to the dying teenager.
In the episode "After Hours", where the owner of a club is trying to seduce Danny. "Was that your wife?" she says. "Yeah..." "What does she have that I don't?" "Me." Way to pull a Gibbs-style 'functional mute'!
Nikki's Chinese fortune cookie says, "You will meet a tall dark stranger". Nikki's mom says, "No, you won't". Nikki's grandfather adds, "Not while I have thousands of armed men at my disposal."
The final part of "This Way Out", as Frank makes a rare show of force, with the NYPD patrol cops, ESU and detectives sweeping the Bitterman Projects and clearing out the gang-bangers. To thunderous applause and cheers from everyone else living there.