Digital thermometers: guaranteed precision
A digital thermometer has a sensor that transmits temperature changes to a microcircuit which records them and shows them on a liquid crystal display. If you're looking for the most precise thermometer, a digital model is the right choice for you. Major errors are almost nonexistent, provided the thermometer is correctly positioned.
Reactive strip thermometers: not very precise
Reactive strip thermometers are like bandanas of coloured liquid crystals placed on the forehead. These take about 3 seconds to change colour depending on the temperature. They are easy to use, but less accurate than digital thermometers.
Infrared thermometers: reliable if correctly used
An infrared thermometer uses a sensor to collect electromagnetic radiation from the tissues, and a microprocessor to convert this into a temperature. It is precise, but only if you use it properly. In some cases, for example, earwax can alter the temperature measurement from an infrared auricular thermometer by up to 0.6° C (33° F).
Galinstan thermometers: maximum precision, but slightiy slower
Broadly similar to mercury thermometers, which are now banned in many countries, these contain a less toxic alloy of metals - gallium, indium and tin named Galinstan. They are very precise, but must be left in position for at least 5 minutes.