Probably Rolf, Pecan, and Willow. It all started in my City Folk town.
Pecan was one of the four squirrels in my town, so already I was sick of seeing squirrels everywhere I went. This, coupled with her snooty personality, made me dislike her at first. She eventually grew on me though, and when I made a new City Folk town she was my neighbor

However, when it came to Willow, I didn't realize she actually wasn't that bad until she moved out

I hope someday she'll come back to me in New Leaf~
Rolf was another one who I judged too quickly. He destroyed my flowers and trees for his house, and at first his appearance kind of frightened me xD I love the cranky villagers though, and I grew to love him <3 In a tragic twist of fate, he randomly moved out without warning

I was legit broken-hearted. Thankfully, he was one of my starter villagers in New Leaf! He's never allowed to leave, haha~ I'd say Rolf is probably one of my favorite villagers of all time now :'D