I have found over the years that I personally love the sporting group. Most of my favorite breeds are in it. Least favorite over all would have to be the herding group.
I think the Herding group would have to be my favorite. Least favorite overall is probably the Terrier group (although there are many Terriers that I love!).
Least favorite.... if we take into account the new group divisions then I'll say scent hounds. I really like sight hounds, though, so not the whole hound group.