Will's screaming reaction in "Rendezvous" after Sydney, dressed in a sexy cabaret singer outfit, knocks out the guards that are holding him captive. This is the first time he's ever seen her do this... and the first time he learns she's a spy.
Everything about Jack and Irina having to go on the mission to India together in "Passage I & II", but especially the Bristows playing "nuclear family" at immigration, with Sydney as a gum-cracking bored teenager and Irina kissing Jack when she knows he can't flip out.
Just about any scene involving Marshall.
"Welcome. Don't kill me." (said to Sark, upon meeting him)
When he has to go into the field with Sydney, Marshall adopts the most atrocious British accent imaginable.
Jack talks to an old ally who just met a pregnant Syd at a party.
Woman: Either that's a very convincing disguise or...
Jack: I'm going to be a grandfather. A very young grandfather.
Syd and Rachel are in a car (Rachel in the trunk) which is hoisted up high in the air by a crane.
Sydney: Don't panic, I've been in far worse situations than this.
Rachel: (from the trunk) You have?
Rachel and Sark have to deal with finding out who the other is after their one-night stand, culminating in this bit of dialogue:
Rachel: Just so we're clear... You approached me, you were the one who let me into your room, and afterwards you were the one who wanted to cuddle.
Sydney and Michael are at a club, teasing each other on how they both "like it rough". Sydney then suddenly wonders if they're still on comms.
Jack: Yes, you're still on comms! Right now!
Jack going into the field as a blue collar, goombah-accented repairman.
— a certain Lieutenant Columbo.