Selectivity should not be the primary indicator for choosing index columns. Putting the most selective column first would be stupid if nothing filters by it. The decision as to column order in indexes should be what queries filter by that column. Selectivity is a secondary consideration.

I've linked this blog post again, it's discussed in there or in the posts linked to.

http://sqlinthewild.co.za/index.php/2011/11/11/sql-university-advanced-indexing-indexing-strategies/