There are so many good ones I find it hard to pick out just a few. Mine would have to include:
~ The one where Tiny has just been delivered, with Lord Fear ranting and mimicking Skarkill (not to mention the look on his face when he has to call it "Tiny"). I also like the one where LF berates Julius Scaramonger for selling Duncan the key - his rage throughout Series 5 was so entertaining!
Pretty much anything with Aesandre cracks me up as well.
~ The one in Series 6 that starts with LF having a good old friendly natter to someone! Hilarious to glimpse a whole other side to him.
~ Skarkill wanting to be paid "waiting time". Fear's facial expressions alone are priceless.
~ "There's probably a technomagical fault in the gubbins... if I keep fiddling with it I can usually get the picture back!" "He'd try it on with a goblin if he caught one in a dress." OMG, that whole scene is just pure brilliance from start to finish.

~ Lord Fear's amusement at Sly getting slimed by Grimaldine, heheh!
~ Another Fear & Sly scene from Series 7 really makes me laugh; the one about Sly's boozing and the fake Wand of Majesty. There are so many great moments in that clip, from Sly's excuses to their aborted evil laughter, to Lord Fear calling the Brollachan "my little wallflower" and loads more. Superb.
~ Also from Julie's quest, Greystagg getting mocked to within an inch of her life, bwahaha. "Dragons take you!" "And you, dear. Ciao!"