The most popular type of flame-resistant (FR) shirt can vary depending on the specific industry, work environment, and workers' personal preferences.

However, some of the most popular types of FR shirts include:

1. FR work shirts - these shirts are designed for heavy-duty work and often feature reinforced seams and pockets and FR properties that protect against heat and flame hazards.
   
   
   
   
2. FR henley shirts - these shirts have a simple, casual design and are often made from soft and comfortable FR fabric that protects against heat and flame hazards.
   
   
   
   
3. FR t-shirts - these shirts have a simple, casual design and are made from lightweight FR fabric that protects against heat and flame hazards.
   
   
   
   
4. FR polo shirts - these shirts have a collared design and are often made from breathable FR fabric that protects against heat and flame hazards.
   
   
   
   

Ultimately, the most popular style of FR shirt will depend on the specific needs and preferences of the workers and the requirements of their industry or workplace. It is essential to choose an FR shirt that provides adequate protection against heat and flame hazards and is comfortable to wear for extended periods.