America's greatest living ballad singer is notoriously inconsistent just listen to last year's ghastly reggae experiment, Countryman. But Songbird, a collaboration with Ryan Adams (who produced and lent his backing band) is Willie Nelson's finest in a decade. The sound is burly, surrounding his inimitable lilt with shuddering electric guitars. But the slow and stately stuff really sparkles, such as a weirdly menacing cover of ''Amazing Grace'' and the new Nelson composition ''Back to Earth,'' which proves the old troubadour can still write the best weepers around.

