The fastest motorcycles you can buy at a dealership are deliberately engineered not to exceed 186 mph. To explain: motorcycles come in two basic types. Us Americans are most familiar with the cruising or touring type, where the rider sits upright. Harley-Davidsons are a perfect example, though such bikes are also made by Victory, Triumph, Honda, Yamaha, and others. These bikes can be fitted with large and powerful engines (and/or chopped to reduce weight) and will travel quite fast. Think of this type of bike like a muscle car, say a Chevy Camaro.
The other basic motorcycle is the sport bike, usually called the crotch rocket, where the rider leans forward. These machines are engineered from the ground up to go as fast as freakin' possible, as quickly as possible. They generally boast inline-four engines and not only achieve incredible speeds, but also safe handling and balance at those speeds. When you hear "crotch rocket" you probably think of Japanese manufacturers (Suzuki, Kawasaki, Honda) -- but many of the speediest class of sport bikes now come from the Europeans (BMW, Bimota, Ducati, MV Agusta). Think of this type of bike like a supercar, say a Ferrari Testarossa.
For the most part, cruiser companies like Harley have given up trying to make the world's fastest bikes. Harley does have a line of big-engine cruisers that can really move (look up VRSC), but by and large they have correctly seen that their market and audience isn't too terribly interested in 186 mph (or anything close to it). As a result, there are no Harleys in the motorbiking racing circuit, nor does Harley (or Victory, or Triumph) have a serious racing division.
The sport-bike manufacturers -- every last one of them -- have serious racing divisions, into which they pour millions to engineer the fastest and most efficient bikes around. These companies use the caché they gain from the racing circuit crowds to lure thrill-seekers into the dealership to try the fastest shit on two wheels. But, in 2001, the Japanese and European bike companies came to a gentleman's agreement to limit (by design or using speed-controlling-computers) all of their production bikes to a maximum top speed of 300 km/h, or 186 mph. If you, the consumer, tinker with the bike to get it to reach 200 mph, well, Ducati et al. take no responsibility.
That said, the fasting thing out there is the BMW S1000RR (pictured here). It has a 999 cc inline-four engine (note how those clever Germans made it exactly one cubic centimeter under 1,000!). There are many bikes with larger engines (like the bad-ass Harley VRSCSE Screamin' Eagle CVO V-Rod), but BMW gets far more out of much less. The S1000, when revving at 10,000 rpm, produces an astonishing 78 foot-pounds of torque; at 13,000 rpm, it's cooking 179 horsepower. On a *motorcycle* mind you -- most full-size Honda Accords don't move that many horses. It goes 0-60 mph in 3.1 seconds (I mean, holy shit!), and can reach its top speed in 19 seconds. It has a wet weight of 458 lb and is actually quite reasonably priced at $13,000. And, despite the gentlemen's agreement, the S1000RR is so well-engineered that it consistently hits 188 mph on the test track, without any modifications. And it does so with ease. See you on the highway. I'll be that blur roaring past ya.