After getting some experience in Zwift, I realized that there was a badge for 100kph. The goal is to hit 100kph, or 62 miles per hour. For many riders this is very difficult, but if you time your sprint, bike choice, and choose you descent wisely, it will be pretty easy.
Descend the Radio Tower
One option is to descend the Radio Tower, the steepest climb on Zwift. With an average gradient of 13.7% and 150 meters of elevation gain. In order to get to the Radio Tower you must first climb up the Epic KOM reverse, then take a turn near the top of the climb when the route selection comes up. Once you reach the top, I would recommend switching to a TT bike, the most aerodynamic one you have. You can find a list of the fastest TT bikes on Zwift Insider. Start at around FTP to get your speed up as you start heading down the climb. Once the gradient turns to around 12-13%, start you sprint and hold it for as long as you can. Around 2/3 the way down the descent is when you would likely get the 100 kph badge. Good luck!
Descend in a group
Another option is to descend a climb with a group. We all know that Zwift pack speeds are slightly unrealistic, the draft is much more powerful than a single rider. Descending with a group will easily get you the badge, but you need to make sure the group is bigger than just 1-2 riders. Make sure that you are all on road bikes. The best place to descend a climb with a group is likely the Radio Tower or the Bologna TT course. For upcoming group rides/races on a route that goes through the Radio Tower look for group rides/races on routes like Mountain 8, Mountain Route, Muir and Mountain, Three Sisters.