It all depends on price and performance.

Best bang for your buck:

Drillmotors and Scorpion XL or Sabretooth, with the blue wheels (They are really good wheels for feathers), and 18V Nicads for the batteries. NiMh is also an option but more expensive and more maintenance prone.

Higher priced, more performance:

Speed 900 BB Torque motors, own made gearbox, Blue wheels, Scorpion XXL (They are also 2x25A Gary) or Sabretooth 2x25A or even Sidewinder, on 24V Nicad or NiMh.

Riduculous amount of torque and speed:

2 Astro Cobalt 90 motors, blue wheels, custom gear reduction, 24V NiCad, Sidewinder or small Vantec speed controller. Good luck keeping that on the floor. It is what I refer to as the obscene power drive for feathers. But it is not cheap.