In mixed dentition analysis, which tooth is used for classification?
The first permanent teeth to erupt into oral cavity is permanent first molar. 
From that phase of dentition, it is called as mixed dentition.
Once the first permanent molar erupts into occlusion, it is used for the classification of malocclusion.
Primary 1st molar
Primary 2nd molar
Permanent 1st molar
Any of 2 or 3