I don't like the term sleeper. Its pretty difficult to keep someone who is knowledgeable about cars from figuring out if a car is modded or not.
Also, one man's sleeper is another man's obvious performance car. The less knowledgeable V6 Mustang owner might be completely suprised when he gets beaten by a VW GTI or a tuned Civic, thus making the GTI or the Civic a sleeper in his eyes, while a Ferrari owner might be completely caught be suprise by a Buick GNX, making it a good sleeper as well. Because of this, there is no perfect or best "sleeper" car.
Just about any car can be a sleeper if you want. Just find something that isn't commonly used. By now most of us have seen the videos of the Dodge Caravan beating up Camaros, or the VW Golf (not a GTI) putting down a thousand horsepower.
Finally, whats the point of a sleeper anyways? Does it really matter if you catch someone by suprise? Just because a Corvette owner doesn't expect a BMW M5 to keep up with it doesn't make the Corvette any more or less of a car. At a drag strip or a track, people expect competition from anything and everything, so it won't really help to have a less common car.
My advise is that if you want a high performance car, ignore the looks and find something that is easily capable of big numbers. Who cares if people expect a WRX or a 240sx to be modified? If they are actually racing they will get the same result.