Bugatti Veyron Super Sport: 268 miles per hour (431.3 kilometers per hour)
Some Bugatti loyalists might vehemently disagree with this number-two ranking. Long story short, Bugatti's Veyron broke the record in 2005, just a couple months after the Koenigsegg CCR's triumph, but the run wasn't certified by Guinness. It was, however, good enough for some fans -- and anyway, Bugatti made it official later. Or, rather, they tried. When the Veyron finally made a Guinness-sanctioned attempt in 2010, the car achieved a top speed of 267.81 miles per hour (431 kilometers per hour). Then the Guinness officials found out that the car had its speed limiter removed, which violated the "no modifications" rule, so Guinness stripped Bugatti of its title. In the end, the Guinness Book of World Records decided the altered speed limiter didn't change the Bugatti Veyron Super Sport's mechanical abilities, so the car was awarded the "world's fastest" designation. At least by Guinness. But not by HowStuffWorks.
Oh, and a notable footnote to Bugatti's record-setting attempts: They got to use the Ehra-Lessien oval test track owned by Bugatti's parent company, Volkswagen. It's a lot less complicated than using a straight runway or drag strip, allowing the car to build speed rather than making the driver worry about stopping in time to meet the end of the pavement.