The most popular lens selections by far are 2.8mm and 4mm. These give the widest angle of view.
The below image gives an indication of the angle of view of each lens.
2.8mm gives around 90 degrees viewing angle.
4mm on average gives around 75 degrees viewing angle.
Things to note:
the more narrower the lens, the focus is further away. Eg. typical 4mm lens is focus 4-10 metres. typically 2.8mm is 1-6 metres. This means that for areas like the front door where you are capturing someone 1-4 metres away, 2.8mm is ideal.
Sides of homes, 4mm may be more ideal.
Driveways which are say 10 metres long will get more ideal image with 4-6mm if what you're trying to see is closer to the entrance of the driveway.
Lens Simulator - http://polarisusa.com/pvi4
If you want another explanation, please watch the below:
