Which type of refrigerator/freezer thermometer is most accurate?

I'm not necessarily looking for a specific brand (though that might be helpful in addition to my primary question), but I would like to know what type of thermometer is most accurate for use in the refrigerator and freezer, bi-metallic, liquid (spirit) filled or digital and why is this choice better than the other two? I've tried researching this online and can't find much to help me out. I have two bi-metallic thermometers and they don't seem to be very accurate at all, but this may be due to the quality of these specific models (they were only $5 a piece) and not indicative of bi-metallic thermometers in general. I know the spirit filled (liquid filled) are the oldest, but that doesn't mean they are less accurate, necessarily. Likewise, I know digital is the newest technology but again, newer doesn't imply "better." Thanks in advance for your answers. :)