The most frequent tooth to be impacted is :
Impacted tooth :

Is the tooth that has failed to erupt completely or partially to its correct position in the dental arch and its eruption potential has been lost.

Order of frequency :

Mandibular 3rd molar (i.e. Lower 3rd molar).
Maxillary 3rd molar (Upper 3rd molar).
Maxillary canine.
Lower third molar
Lower canine
Upper third molar
Upper premolar