The best thermometers for making soap are either the candy thermometers or the digital infrared type.
Getting accurate temperatures when making soap means the difference between having your soap turn out properly, or a soap that you'll have to throw away. For the purposes of making soap, the ingredients you will be working with will be room temperature then up to a temperature of 185 F degrees.
The candy thermometer, also known as a deep fry thermometer, is good in that these will measure from 100F to 400F. This variety is encased in a round glass or pyrex type that comes with a clip for attaching to the side of your pot. Often identified with a large red ball attached to the clip. You won't need to measure anything higher than 190F but you can also keep checking the temperature to ensure the temperature of your oils are not going too high. All oils have a flash-point, meaning a temperature where the oils will burst into flames. There should be a flash-point listed on your oils packages and containers.
Inside these candy-type thermometers is either mercury or alcohol, which rises to indicate the temperature. Get the alcohol version in order to eliminate any toxic exposure from a broken mercury filled thermometer. Keep your special soap making thermometer with your soap making supplies and have a separate one for food and cooking purposes.
When using your candy-type thermometer, clip it to the side of your pot without allowing the bottom of the thermometer to touch the bottom of the pot. At my store, we had a variety of lye-water containers and some of these containers had high sides and the thermometer could not clip and reach the liquid. In these cases, I removed the clip and let the thermometer float on the top of this caustic liquid, later we would use bacon tongs to remove it. If you are budget conscious, this would be the least expensive and best thermometer for making soap.
Also, if you need 2 or more thermometers, try to stay with the same company brand. We tested a couple of different brands we had and found a 5F degree difference between them.
A digital infrared is probably the quickest and safest way to check temperatures, but it is also the most expensive. These start at about $40.00, whereas the candy thermometers are approximately $3.00 each.
The digital thermometer uses infrared to detect temperature, and because of its simplicity, you can measure numerous items in seconds. Another top feature is the accuracy, and having accurate temperatures helps to eliminate some of the variables that could ruin your batch of soap. Cleaning is not needed with these types of thermometers as they never need to touch the liquid. It is these benefits that make the digital one the best thermometers for making soap.
Most of these digital types are small and easy to use, have liquid crystal displays, read in both Fahrenheit and Celsius and some even have holsters and cases.
If you are undecided on which are the best thermometers for making soap, and if you will be making lots of soap, use the candy thermometers to start with. Then later if soap making looks like a great little business for you, then invest in a decent digital thermometer.