YFZ450-R. The YFZ is probably the most potent (commonly-seen) ATV on the market in stock form. It has good shocks, a well-engineered frame, and an effective EFI system on the "R" version. The engine is pretty stout right from the factory. Add a pipe, fuel tuner, and a tether kill switch, and you are just about ready to hit the track.
The Can-Am DS450's are nice, too, and will give the competition a run for its money.