Parked next to the dry dock. Getting on this bike triggers the "NRG-500 Challenge".
Its high power can lead to unwanted "wheelies" if you are not careful with the throttle, which may affect steering at high speeds. Where to Find the NRG-500 (No Cheats Needed)
It offers the best acceleration, braking, and cornering in its class.
Look on the second and top floors of the multi-story parking garage.
For a visual guide on exactly where to find the NRG-500 at the very start of the game, watch this tutorial: