Digital aquarium thermometers are considered by many to be the most accurate of all styles, although that can vary greatly by brand. Interestingly enough, the price isn't always an indication of quality. Check current customer reviews carefully before making a purchase. Also don't forget to calibrate your digital aquarium thermometer using the instructions provided with the product. I've included some high-end and more moderately priced models in this list.
This digital model does not have a probe that is permanently affixed inside the tank. Instead it is used to take a quick temperature check. Useful for testing water in a bucket to be used for topping off or water changes. Also useful to verify the temperature in tanks with a permanent thermometer, or to do routine water temperature checks on tanks with no permanent thermometer. Display is easy to read, and is available in models for Celsius or Fahrenheit scales. Has a calibration check switch.
Comes with a suction cup to attach the meter to the side of the tank, and a probe connected by a three foot long wire. A switch allows the digital display to be changed between Celsius and Fahrenheit scales as needed. I've had good results myself with this product, however I think the key is to calibrate it to ensure it's set accurately. There are reports that Coralife does not manufacture this product, but vends it out to a manufacturer that sells the same product without the Coralife tag. I have not been able to verify the accuracy of that report, but I have seen non-branded digital thermometers that look exactly like Coralife. Either way, you should always shop for the best price possible.
This is one of the digitals that has had so-so reviews from consumers. The issue seems to be failure after a period of time. I have not used one personally, but due to the number of owners who have reported problems either to me or to customer review sites, I would be careful about using this product in a tank in which temperature is extremely critical.
This is a more expensive model that is designed to be used for multiple tanks at the same time. It comes with a single sensor, but up to three additional probes may be added. The digital display connects wireless to the probe(s), and can be toggled between Celsius and Fahrenheit as desired. Both the sensor and display run on AAA batteries that are not provided. I have not used this meter myself, and reviews are widely mixed. Those who report they like them, swear by them. Most of the complaints I've heard personally were due to issues with the wireless receiver, or cost of the batteries for both components, rather than accuracy.
This is another model I have yet to try, but it looks interesting. Instead of a probe in the tank that connects to a digital display outside the tank, this thermometer is completely submersed. My primary concern would be leakage, as the device has batteries inside it. If anyone has used this model, please post your comments.