What is the most fragile tooth??

Incisors: The front teeth. Incisors. Most prone to injury.

Fragile teeth: This is a difficult question to answer. The most common teeth to fracture are your upper and lower molars. When it comes to trauma, your front teeth are more likely to get broken because they stick out the farthest. In my opinion, all teeth are fragile, don't use them as tools (i.e. cracking nuts, opening/biting things other than food) and wear protective equipment when playing sports.

Lower incisors: Lower incisors are the smallest teeth in the mouth and from that standpoint the most damaged by decay or trauma. Teeth in general become more fragile from fillings. The larger the filling, the more prone the tooth is to fracture or break. That's why is so important to restore fillings early when they are small.