You said thermometer, not controller. Just to read temps? In that case, most affordable thermometers have around the same accuracy, though the analog cheapies may have less accuracy. For this application, I would go digital with remote probe.

The Taylor therms from the Bean Farm are very good, around $13 or so. Zilla makes a decent small digital thermometer. For incubating eggs these will be fine to indicate your temps. Its your temperature control that you have to make sure is reasonably decent.