You know you shouldn't be laughing, but you can't help it, can you?
The Punchline to Episode 5: Hypnotized
The start of the session qualifies too.
Joker: If this works, and I wake up, and my pants are off..."
Dr. Hugo Strange: You will not do anything under hypnosis that you would not do in a concious state.
Joker: Like I said, if this works, and I wake up, and my pants are off...
During the cafeteria scene, Harley is ranting about how letting inmates wear masks is a negative step in their recovery, and how it's only a matter of time before it effects things they do in everyday life... and the Scarecrow comes in, sits down, and struggles to eat a sandwhich while wearing his mask. Even better, Harley tries to haltingly continue her rant, and the Joker just stares at him the whole time.
And then Scarecrow notices there's a bite out of his sandwich. How did that get there?
Episode 15: Get Me to the Church On Time - Everything with Ted the Bum.
"You just lost your gun priviliges! If you had hit Pearl, you could have kept it."
One episode later, Ted's gun privileges get re-instated just as the cops start opening fire.
Episode 8: BRB - "It always comes down to making a choice. One or the other. It's the fork in the throat— road, the fork in the road. [Beat] Henry's the one with the, uh, fork in the throat." Perfect Black Comedy one-liner, perfectly delivered.
The cartoonish way the Joker tried to prop himself up against the door Batman is trying to get through.
The entire preformance of the irish/scottish priest.
Further Evidence 4 shows us that Lyle has gone completely bonkers, working with Scarecrow and Dr. Arkham under the name Lock Up. His banter with Harvey Bullock's girlfriend is particularly amusing.
Harvey getting rid of a 911 call by pretending to be animal control.
The teaser for Further Evidence 5 is Dr. Tommy Elliot leaving a voicemail for Bruce Wayne. He talks about the many injuries (bullet wounds, knife wounds, animal bites) he saw on Bruce during his surgery, and it is evident that he found out Bruce's alter ego... until he cautions Bruce on the dangers of recreational drug use.
The end of the Joker's hypnotherapy session with Dr. Hugo Strange.
"This is Steve. Say hi, Steve."
"And here's a lovely picture of your wife...I'm in the wrong office."
The end of #15; Arkham sputtering that there was a shotgun in the trunk the whole time they've been riding around. Including when he was in there.
At the beginning of #15, Joker asks Arkham if he's grumpy.
Arkham: Yes. Yes I am grumpy.
After escaping the Joker in Episode 16, Dr. Arkham wildly runs to a cop car he encounters while running through the streets. His exasperated reaction when he sees who's driving the car is just priceless.
Also in 17; "Is that video still up? Damn you, 4479!"
All of 4.5. Joker is being interrogated by Arkham about chocolates that were left in his office, and Joker looks honestly confused that they haven't been opened. Then, once he leaves, Joker holds up a series of cards telling people to send cards saying "Happy Valentines Day!!! From Youtube.com/The Joker Blogs".
The cards sequence is brilliant, because he doesn't say a word through the entire thing. First he holds up the things required to do this (A pen, paper, an envelope and postage), gives the message above and address of Arkham himself before holding up a card that says "I hope you send lots!!!" The whole time, he's just lying on the table like a little kid.