This is the heart of your tool set. Unlike wrenches or sockets, there is no different tool for each size fastener. This tool will take the brunt each time you use a socket. This is no place to pinch pennies. There are three primary considerations with a ratchet. The first, is how well it fits in the your hand. Some ratchets use a basic steel handle with a shape designed to feel comfortable in the hand. Others use a plastic handle so that when working in a cold shop or hot engine, the heat is not transferred to your hand. A few use a rubber-based handle that offers tremendous grip and is soft on the hands. Try out different ones if you can to see which you prefer.
The second large consideration is how well the drive stub fits the socket. The first thing I noticed when I changed to a premium ratchet was how loosely the same socket fit on the old one, and how snugly it fit on the new one. Sloppiness in this area leads to wasted effort and torque. The third major area of concern is what we call "swing angle". Basically it means how far you have to swing the ratchet in one direction or the other to get it to click. This is directly proportional to how many teeth are in the tool. The more teeth, the smaller the swing angle. If you imagine yourself working in a confined area where you can only move the ratchet back and forth a little at a time, you'll get an appreciation for the importance. The best wrenches have 72 teeth and thus can ratchet in 5 degrees of swing. If you can afford it, settle for nothing less.