If by 'people' you mean all of us living in the western world then it is clear that the Gregorian calendar is the most widely used. To calculate 'mostly used' you would have to use the populations of the various countries that depend on each calendar. The Gregorian is the calendar used in the international standard for Representation of dates and times.
Because it calculates the year from the time it takes the Earth to circulate the sun, it is the most accurate way of calculating the passing of time, making sense of the term A Year. the Gregorian calendar aligns with the movement of the Earth around the Sun and also corresponds to changes in seasons at different latitudes.