The M Frames. IMO they were one of the greatest pairs Oakley ever made, and revolutionary for their time. Too bad they hardly sold in their 20 or so years of existence...
On a serious note, however, I'd have to agree with everyone's responses so far. The Plate, Hatchet, Switch, and Unknown were all really cool pairs, each for their own reasons. Personally, I always liked the Scar, even though they sold poorly at retail. Incorporating a new design concept at the time (combining X-Metal with O-matter) along with CAD/CAM design elements and a wide range of color options made this pair a winner in my eyes.