Depends on what your criteria is for Top Of the Line.
Tunability, Power - low end -mid range- topend, cost,
reliablity, driver skill ect ect...

How about your track or tracks you plan on racing on.
Long, wide and smooth or tight and bumpy?
High grip or low grip surface? Hard on tires
or easy on them?

What about your skill level? Be honest with yourself
if you can't handle the power of a top level engine
you will be faster with one of the cheaper sport mills
the NovaRossi N21 comes to mind.

Truth is I can't handle all the power my V-spec makes
with the 9mm restrictor.
We ran the first round of our state pro series yesterday
and I qualified 4th for the Sportsman Buggy A main.
About two corners in to the 30min main I spun the
center out of the rear brake rotor, after that I
was plain out of control. Several hard crashes later the
left front steering knuckle went and I was done.

If you had asked me on the stand at that point if I
would have liked to have a smoother easier to control
motor the anwser would have been yes. Add to this
that the track had become very abrasive on tires
means you will finish mains on slicks if you don't
have proper throttle control.

So give us a littile more info.

1. Budget for motor and pipe.
2. Driver skill level.
3. Track or Tracks layout.
4. How long do you expect or want it to last.
5. How is your tuning skill.