The most helpful place to donate clothes is to a homeless shelter or church that gives out clothes.
The Salvation Army and Goodwill are also good and helpful places to donate clothes but if you want to just donate the clothes so they are given to other people then a church or homeless shelter would be a good place.
Children and family shelters also are in need of clothes and even some hospitals have programs that supply clothes to the homeless or needy.
If you have a soup kitchen that feeds the needy they also sometimes will accept clothes to be donated to other people in need.
The VA also has programs in some areas that accept donated clothes or check with The Arc, Soles4Souls, or Project G.L.A.M.