An excellent study has been done of oil filters. There were some suprising conclusions:
The Best:
AC Delco
Purolator (same as Proline)
STP
Mopar
Mobil 1
Bosch
The Worst:
Fram
Penzoil (made by Fram)
Quaker State (made by Fram)
See the full study at:
http://minimopar.knizefamily.net/oilfilters.html