What liquid is advisable to use as liquid inside the thermometer mercury or alcohol?

Rather ironically, it depends on the temperatures involved. Most thermometers use Mercury. Partly, this is because the silver colour of the mercury make it easy to read, but mainly because they work over a useful range of temperatures for everyday use.

However, one problem with mercury is that its melting point is -38oC; so below that temperature the thermometer would freeze - this makes it unsuitable for use in very cold environments.

Alchohol thermometers (usually ethanol is used) work from around -115oC, so they are used in low temperature environments - the disadvantage with alcohol is that it vaporises at 78oC so it isn't useful in very hot environments.