I think that a sleeper is the kind of car that makes you go "WTF!?" when it clearly gets the best of you. So with that definition in mind, I think that wagons and sedans with conservative looks, respectable handling, and lots of horsepower make the best sleepers.
Normal cars with big engines (Toyota Camry V6) don't count in my book. The car needs to have received a bit of love and special attention from the factory. An enthusiast would actually have to enjoy driving it.
My top votes go to:
1. Subaru Forester XT
2. Volvo 850R / S60R / V70R
3. Mazdaspeed 6
4. Volkswagen Passat with W8 engine (anybody else remember those?)