Every which way but loose! Right turn, Clyde. :)
Personally my favorite might be Where Eagles Dare, but that might also just be out of nostalgic reasons. The Unforgiven is great, and the Dirty Harry series is always enjoyable. In the line of fire seems to slip under the radar I suppose, a really good wolfgang petersen thriller. Also special mention HAS to go to Heartbreak Ridge, love it!
As a director I've thought his films have been quite boring a lot of the times, really long and so on. However I enjoyed Gran Tornio, Flags of Our Fathers/Letters from Iwo Jima were fantastic and the new film Invictus was too. Changeling was pretty good as well, like some of his other films just maybe too slow-paced for me but really good in the end.