KerbalX

We’ve done 2km/s, we’ve done 2.1 km/s, we’ve even done 2.11 km/s. Hell, my last speed record got upwards of mach 6! But that’s nothing anymore. Behold… the speediest (probably, I didn’t care to check) of the speedy

Powered by a HyperKraken Drive, this aircraft is capable of accelerating at roughly 15Gs. Its top speed increases as it gains altitude, it can fly over Mach 15 below 1km above sea level.

To control throttle, zoom into the craft. The active docking ports are on the CoM, so you should zoom right into them. Use the landing legs for takeoffs and landings. To keep from overheating, it is advised to fly at less than full throttle, or to turn reentry heating down in difficulty settings.
The SDESZZPwVTOLC is not only incredibly fast, but also customizable. You can swap out the landing legs for conventional landing gear if you like having a difficult time landing, or you can swap out the wings for bigger ones if you want to pull more Gs.

Because the propulsion doesn’t need such silly things as air or your approval, you can take this thing into space. That’s right, it’s an SSTA that can perform massive burns super quickly.

To see a graph of top speeds at different altitudes*, click here. Mileage may very. Data points taken every 2.5km, and at 50m.
*To measure top speeds, I increased gravity by a lot. Doing so decreased drag, as this thing goes so fast it needs to constantly pitch down to avoid climbing rapidly. Data valid for the given configuration only, and not for any configuration with different wings or landing gear