The narrator telling about the events, especially in the first episode of the 1st season. This is courtesy of the English described captions in the DVD. Even though it's trying to help, just listening to it and watching most of the scenes sounds funny since they're mostly Captain Obvious.
Des and Jake's interaction is usually made of funny:
Jake: Why is it that whenever I talk to you, I want to punch you in the face?!
All of this has nothing on Jake and Malachy's snarkfests, however. Good grief.
Take, for example, the instance when Jake nearly gets himself killed (again). Malachy starts railing at him, and Doyle retorts "I'd ask you to come down and help me, but of course you'll never get your walker down here!"
There's another instance when Malachy is just getting out of jail, and Doyle asks him how it went. Malachy's response? "Dandy, they're talking about naming a wing after you."
"What are you two whispering about?" "Nothing. ...Des has fleas." "I don't have them anymore!"
Jake accidentally holds up a bank in "High School Confidential", resulting in a media frenzy and the entire police force arriving at the station.
How Des scares off an attacker: runs at him screaming and waving his arms like a windmill.
Jake: What the hell was that?!
Des: ...the element of surprise!
Jake and Maurice butting heads when they were telling the RNC about what happened to Leslie and Hood. Maurice makes Jake look like a wuss, idiot and a chicken. Jake makes Maurice looks worn, tired and not neat with his clothes. In addition, Jake looks tough and confident in his stories while Maurice looks cool and sophisticated in his stories.
Des' reaction when he sees Jake after the latter missing for two months in the season 5 premier. Doubles as a moment of Heartwarming.
Des: What's up.
Jake: "What's up"?!
*Des runs at Jake and hugs him*
Des telling Tinny that he's a grown man with his own apartment now, and they shouldn't need anybody's permission to date each other. Cue his phone ringing.
Des: Oh god it's Jake he's on to us!
After Jimmy (once again) gets hit by a car:
Wolf: How you doing, Jimmy?
Jimmy:*Slurring his words and barely able to keep his eyes open* I'm going to cover Jake with butter and tickle him to death.
Jake and Leslie try to have a serious conversation in the surveillance van while the couple in the house Jake is watching start having sex. Jake manages to turn off the picture, but can't figure out the sound. Even ripping the speaker out of the socket doesn't stop the moaning from being broadcast (loudly) into the van.
In the same episode, Des gets into a staring contest with a dog.
Des breaking out of the hospital to help Jake in the series finale, but forgetting his pants. Especially funny as they're sneaking into the courthouse and he's trying to hold his hospital gown closed in the back.
Mal's "distraction" to allow Jake and Des to sneak into the courthouse— repeatedly attacking Walter— is also hilarious, as is Hood's bemused reaction.