I always use TWO of the liquid crystal sticky-on ones. One at the top of the tank, at the waterline right under the lights, and another at the bottom of the tank and as far from the warm water flow from the heater as possible, e.g., behind a big piece of rock or wood. This way, I get to see the spread of temperatures, from the highest to the lowest.
I did have a digital one (an £18 Marina one that happed to come free with an external filter). Total rubbish. The battery died after about 6 months, and nowhere around here has replacements, and the liquid crystal display began leaking shortly afterwards, rendering the thing useless anyway.
In my opinion, the best thermometers (just like the best nitrite test kits) are your fish. So long as they are behaving normally, I don't check either water chemistry or temperature.
Cheers,
Neale