Always been a fan of poison types starting with the massive variety of them given right from the inception of the series. There is a poison type for every role. Koga had it right, you can completely destroy without brute force.
Them aside, Bug is my second favorite for their quick growth and general coolness. I just love the designs of most bugs Volbeat and Illumise aside. Most are highly specialized but great at their jobs, and just generally fun to use.
Ground is pretty awesome too and always seems to play into any themes I am running, so it comes in at the honorary third place.