正如@juharr在注解中提到的，可以按常量分组，然后取每个值的最大值

var maxYearVolumes = volumes
    .Where(v => v.YEAR == maxYear)
    .GroupBy(v => 1)
    .Select(g => new {
        Jan = g.Max(v => v.JAN_VOLUME),
        Feb = g.Max(v => v.FEB_VOLUME),
        Mar = g.Max(v => v.MAR_VOLUME),
        Apr = g.Max(v => v.APR_VOLUME),
        May = g.Max(v => v.MAY_VOLUME),
        Jun = g.Max(v => v.JUN_VOLUME),
        Jul = g.Max(v => v.JUL_VOLUME),
        Aug = g.Max(v => v.AUG_VOLUME),
        Sep = g.Max(v => v.SEP_VOLUME),
        Oct = g.Max(v => v.OCT_VOLUME),
        Nov = g.Max(v => v.NOV_VOLUME),
        Dec = g.Max(v => v.DEC_VOLUME),
    })
    .First();

如果您使用Linq-To-Objects并且担心性能问题，可以使用.Aggregate，但是编写一个普通的foreach循环可能更容易。