Sam and Gene's synchronized desk-leaping from the pilot.
Not to mention Gene's introduction:
Sam: All right, surprise me. What year is it?
Gene: A word in your shell-like, pal. (slams Sam up against a filing cabinet)
Sam: Big mistake. (shoves Gene away)
Gene: Yeah? (sucker-punches Sam in the stomach) They reckon you've got concussion. Well, I couldn't give a tart's furry cup if half your brains are falling out. Don't ever waltz into my kingdom acting king of the jungle.
Sam: Who the hell are you?
Gene: Gene Hunt, your DCI. It's 1973, almost dinner time. I'm 'aving 'oops.
Sam to Gene in 2.05: "I can just about handle you, driving like a pissed up crackhead and treating women like beanbags. But I'm going to say this once, and once only, Gene. Stay out of Camberwick Green!"
In 1x06, during the hostage situation, Gene is shot three times, point blank. The thing is? He's got three flasks in his jacket.
How, at the end of episode 2.01, Sam gets Tony Crane sent to an asylum, therefore preventing a future murder AND keeping his cover. AWESOME.
Annie Cartwright's entire performance as "Cherie Blair" in episode 2.04.