The handyman prefers wood, but wood has to be stained or painted, just as most outdoor wood surfaces must be painted. Wood doors are also heavy. Metal, usually steel, has its strengths and weaknesses. It is light in weight, but has to be kept painted, and if rust occurs, painted is more difficult. Metal doors can dent, and even paneled metal doors are not attractive, which is strictly an opinion.
Flush doors are practical, but pretty ugly.
Again, that is an opinion. New are fiberglass doors, and relatively new are hardboard paneled doors. These must be stained or painted, but are light in weight. So take your pick. The flat roof on my two-car garage is tar and gravel.