There are so many baby thermometers out there, the best one might be difficult to pinpoint. In general, most physicians recommend digital thermometers, not ones containing mercury. You can get both oral and rectal, but rectal tend to be more accurate for babies. They are quick to use and give accurate readings. Axillary, or underarm thermometers, must be held under the baby's arm for ten minutes. Oral is reliable, but because it takes about two minutes, you can end up struggling if you have a fidgety baby. Oral thermometers are better for children at least 3-4 years old.
Some trusted brands of baby thermometer include: The First Years, but ask your paediatrician if they have any recommendations.