A9 Small Nanny Cam

The best spy camera is small, unobtrusive and easy to use. While home security cameras can do exactly the same, they're usually more obvious and easy to spot. Spy cameras need to be hard to detect and easy to disguise in discrete places. 

Other than size and how easy they are to hide there are a few other things you should be looking out for. Firstly, you want the camera to have a wide field of view so It can see as much of its surroundings as possible.
While resolution is really important with most cameras, it doesn't matter so much with spy cameras as you're more interested in the content you're recording rather than the quality of the image.