What battery is lightweight and has a lot of power?
LiPo batteries are lightweight and have very high power density and output current. If cost is no object, they would probably be your best choice. However, be warned: if charged wrong or used improperly, they get angry. And you won't like them when they're angry.. 

They're also quite expensive, and require some fairly complex power electronics to reach maximum potential. If you're controlling a brushless DC motor, you can get neato off-the-shelf ESCs which will let you drive them like a servo motor (with a PWM signal)... but BLDC motors tend to be much higher RPM than you'd want for a power scooter, so you'd have to gear it down.