Deeeefault
edit: here's some fun inaccurate stats, because a majority of users either just use the default skin or use the skin chooser at the bottom of the page (which sets the style in the users' cookie) :
Code:
mysql> select user.userid as users, style.title from style left join user on (user.styleid = style.styleid) where style.displayorder!='0' group by title;
+--------+---------------------+
| users | title |
+--------+---------------------+
| 14 | Ass-kickin' Style |
| NULL | Deefault |
| 87 | TNL Classic 2 |
| 19 | Your Boss Hates You |
+--------+---------------------+
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