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.
Just about any scene involving Marshall.
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.
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.