IMHOP, the best type of thermometer to buy is a mercury based one, they tend to be more accurate. If you wish to record the Max and Min temperatures each day then a Max Min Thermometer is a must, unlike the standard thermometer, it has a "U" tube with a temperature scale on each "leg" and above the mercury in each leg is a tiny metal pin. These pins are set to the level of the mercury (each day) with a small magnet (usually supplied with the thermometer) As the mercury expands one of the pins is pushed whilst the other is left at the last recorded MIN temperature, the pushed one indicating the MAX temperature. The situation is reversed when the temperature falls.
You can also get this type of thermometer which works on a bi-metallic strip principle these can be purchased at B&Q for about £7.00 they are not as accurate though, but are suitable for general usage