Gen 1: Ninetales (She has been my favourite pokemon since I first saw it. However, I'd like to give honourable mentions to Rhydon, Arcanine and Wartortle)
Gen 2: Honchkrow
Gen 3: Gardevoir
Gen 4: Staraptor (She's actually not that pretty, but I fell in love with my shiny Starly (and evolutions) in Platinum and love this pokemon ever since)
Gen 5: Krookodile
Gen 6: Heliolisk
Gen 7: Mudsdale