grinningest (**Adjective**: Describing someone who is smiling broadly or showing a wide grin; often used to indicate the superlative form of "grinning.")
1
Most cheerful in appearance or demeanor.
2
Exhibiting the widest smile possible.
3
Showing maximum delight through facial expression.
4
Radiating happiness with a broad grin.
5
Having an infectious and joyful smile; very happy.