Where is the correct place(s) in twentysixteen’s header.php to add a Google webfont?

You can place it in header.php between starting <head> and closing </head> tags and it should work properly.

Or perhaps a better solution would be to enqueue the style in functions.php

function google_fonts() {
$query_args = array(
    'family' => 'Sigmar+One'
);
wp_register_style( 'google_fonts', add_query_arg( $query_args, "//fonts.googleapis.com/css" ), array(), null );
        }

add_action('wp_enqueue_scripts', 'google_fonts');