Mine has to be "Maud's muggers", the three psycho granny's that ambush you in freeside. Everything about them I found hilarious; their names being 'irate Ida' ''rancourous Ruth' and 'malefic maud', and that they try beating you to death with rolling pins and such things.
I also enjoyed one in OWB, the mini deathclaw called stripe that comes out of gabes dog house in Higgs village. That is until I found out it does just as much damage as a regular one, anyway... :/