What movement of the thumb would be most affected by lesion of the median nerve in the cubital fossa

flexion

Explanation:

The flexion is a movement of the body part that is described the bending of an elbow, or the clenching a hand into a fist, etc. where the angle between any segment of the body and its proximal segment decreases.

In simple words, the flexion is a movement which decreases the angle between two body parts when they are moved.

The flexion occurs in the sagittal plane.

Thus, the movement of a thumb that would be mostly affected by the lesion of median nerve in cubital fossa is the flexion movement.