I did this once as well with all the crappy thermometers I had. I finally found one that's accurate and I'm sticking with it until it inevitably breaks next week.
Well the digitals looked to be about 0.5-1 degree lower than the mercury thermometer. The 2 mechanicals were the furthest off, the big one was off nearly 10°. I didn't realize you could use the nut on the back to adjust them, which was one of the reasons for doing this