The superlative form of the adjective 'ruddy', meaning very red in color, especially in the face.
The word 'ruddiest' can make people think of someone who has been outside in the sun for a long time or someone who is embarrassed.
Examples:
Her cheeks were the ruddiest I had ever seen.
Of all the apples, that one is the ruddiest.
The ruddiest part of the sunset was the best.
He looked like the ruddiest tomato in the garden.
After playing outside all day, my face was the ruddiest it had ever been.
The ruddiest face I ever saw was on a clown at the circus.