The best would be a normal/standard angle of view lens. That would be 35mm for APS-C dSLR's and 50mm for full frame. Personal preference though depends on how close or how far you want to be from your subject.
The range of focal lengths then goes from normal/standard to short telephoto. For APS-C dSLR's that would be 35mm to around 55mm. In full frame it goes from 50mm to 80mm. Some photographers still want to go further so it's not unlikely to see 100mm used in portraits.
How much of the subject will be in the photograph is also considered. For whole body portraits, a shorter focal length lens is better. For close-ups, a longer focal length is used.
Wide angle lenses are avoided because of the distortion levels they bring.
So where does the 18-55mm kit lens stand in terms of portraits? Almost perfect. It has 35mm to 55mm. If you know exactly how to use it, you can get great portraits with the kit lens.
For blurring the background, see this.
Now you know!