I like most of the random encounters in fallout 1 & 2. especially the Bridgekeeper. just playing through either game with a low intelligence character was good for some laughs. In fact there might be too many to pick a "funniest"...
it wasn't scripted but in fallout 3 threw a grenade at a raider in the ruins of bethesda, he did a cartwheel way up into the air and landed on a high beam doing the splits, dead of course. I laughed for a good while over that. then realized i couldn't loot him. small price to pay.