Normal Types -- they're anything but. They're my favorite types because many of my favorite designs, including Skitty and Delcatty, are Normal. Mammals happen to be my favorite class of animals, too, and of course many Normals are mammalian. I also like the sheer diversity of the moves they can learn - I can teach my fluffy cat to bring in a blizzard or have my three-tailed bull let loose lightning from the sky. You get tons of options. Not to mention many Normal moves themselves are awesome, such as Hyper Beam, Crush Claw, and Giga Impact, just to name a few.
Other favorites of mine are Grass, Ice, Dragon, Dark, and perhaps Ground and Fairy.
My least favorite types, I guess, are Fighting and maybe Water and Bug.