This snippet will display a table of users ordered by the percent of the site's content (nodes and comments) that they authored.

$limit = 10; //The number of users to display.
$round = 1; //The number of digits after the decimal point to round to.  On larger sites, use larger values.
$uids = db_query_range("SELECT ROUND((cn + cc) / (SELECT (COUNT(n.nid) + COUNT(c.cid)) percent FROM node n LEFT JOIN comments c ON n.nid = c.nid) * 100, %d) percent, n.uid FROM (SELECT COUNT(nid) cn, uid FROM node GROUP BY uid) n LEFT JOIN (SELECT COUNT(cid) cc, uid FROM comments GROUP BY uid) c ON n.uid = c.uid GROUP BY uid ORDER BY percent DESC", $round, 0, $limit);
while ($u = db_fetch_array($uids)) {
  $uid = $u['uid'];
  $pc = $u['percent'];
  $user = user_load(array('uid' => $uid));
  if ($pc > 0) {
    $rows[] = array(theme_username($user), 'percent' => $pc);
  }
}

$header = array('User', 'Percent of Site Content');
print theme_table($header, $rows);