Canon EF 135mm f/2L USM
SIx months on from buying it this has become my favourite lens ever, beating my previous favourite (Leica's 4th version of the 35mm Summicron for its M-series rangefinders). I use it for everything, landscapes, townscapes, interesting detail, portraits.
Wonderful image quality; lots of detail; contrasty; subject separation; fast and accurate AF; bright viewfinder; solid construction; unobtrusive in use; ..
No weather sealing; makes all my other lenses look poor (even the 'L' zooms that; when I first got them; imagined could hardly be improved on)