My choice for the prettiest pokemon in Unova is the unconventionally beautiful Volcarona. The kingdom of insects can be gorgeous too! Butterflies, ladybugs, fireflies, beatles, lightning bugs, dragonflies etc. I also just love winged creatures in art from angels to fairies.
This pokemon's dramatic pattern of primary colors practically hypnotizes me. It reminds me a little bit of the Madagascan Sunset Moth.
My top 3 list for prettiest Pokemon in Unova would be Volcarona, followed by Liepard and Serperior (especially the shiny form) for 1st and 2nd runners up.