I have a soft spot for Bug type Pokemon because they make something I'm scared of / dislike in real life really cool looking. Volcarona, Scolipede, Galvantula, Beedrill, Vikavolt, Ariados, Venomoth. <3.
For dual types I find the ghosts (ghost/fire, ghost/grass, ghost/steel) and psychics (psychic/steel, psychic/dark, psychic/dragon) really intriguing type combinations. Fairy as well because the three of these fall into a supernatural category instead of an elemental category. The end result in their design could be anything as opposed to typically fighting=boxer and ground/rock=boulders and water=something swimming and grass=stuff with leaves on it.