I know it's not AGS, but I always enjoyed the subtle humour of Grim Fandango.
Like, when Manny just finishes telling you how much he hates cats, and then you walk into a room full of kitty litter. If you say <pick up kitty litter> he just laughs at you for about 30 seconds.
Or when Glottis finds some equipment washed up on a beach, and says he could attach it to the boat, but first he has to "do some figurin'". He proceeds to stare at the thing and scratch his head for a while. If you say <talk to Glottis>, Manny just says "I don't wanna bug him. He's figurin'"...
Or Lupe. She was hilarious, the way the frenetic music would play as when she talks...