For road, commuting or mountain biking? That's important as they often have specific sole patterns aligned to the shape of cleats and tread. Important as you'll wear out the bottom of the overshoe by walking / putting your feet down too much.
Waterproof? I'm using the Gore City IVs at the moment for commuting. Takes a MTB style sole, has a velco adjustment up the back and a nice, tight-fitting elasticated top.
If you're after something a bit more "aero" then roadie-style neoporene ones are closer fitting – though not totally waterproof.
Gore do a great range of insulated (Oxygen) overshoes for the deepest winter.
Some folk even wear two sets!