What is the most common medieval shield?

Answer 1

Shield shapes evolved over time, so it is not possible to give an answer to your question.

In the period 500 AD to about 1050 AD, round shields were the most commonly used type; then long, inverted-teardrop "kite shields" with round tops were used up to about 1160; then kite shields with flat tops were common up to about 1190; then slightly shorter shields became fashionable until the small "heater" shape became common by the middle 1200s (13th century).

Heater shields remained in use until the 1400s, when the use of shields gradually ended.

Throughout the medieval period, much larger shields called pavises were used by crossbowmen to protect them while reloading; these were free-standing tall shields with a hinged "leg" at the back that could be picked up and carried to a new position.