The best way to do this would be using a taxonomy for city instead of a text field.
Or you can query the database directly.
// replace city below with your meta key
global $wpdb;
$date = date('Y-m-d H:i:s');
$query = 'SELECT DISTINCT meta_value
FROM '.$wpdb->postmeta.' pm
WHERE pm.meta_key = "city"
ORDER BY pm.meta_value';
$values = $wpdb->get_results($query, 'ARRAY_A');
See documentation for $wpdb here https://codex.wordpress.org/Class_Reference/wpdb
