My least favorite is anything that is scattered over the most distance. I don't have much time to devote to this game and so I hate having to hunt all over for stuff (I know some people wish there was MORE to do in the game, I just don't have much spare time). Like, now they tell you what floor stuff is on, but back at the beginning you'd get "wash windows" or even more annoying "water plants" (watering plants by the way was the one that convinced me I needed to make a map) and it could be anywhere on the first and second floor.
As for favorite duties I don't know...maybe feeding the unicorns. I mean, that's all in one room.
Or cleaning the pool. I love walking around on the bottom of the pool and watching other IFs iron and play video games. Or Fluffer Nutter drinking juice underwater, that's just weird.