It's hard to offer specific advise in this case as there is no "best"
weight. The choice depends on a lot of parameters and personal preferences.
However, here are some things to consider. First, if you are going to err,
err on the heavy side. Second, stick with a cast iron or steel flywheel
because they don't warp under stress like aluminum does. I would say your
best bet if you want to spend the money is a billet steel flywheel. I would
recommend getting one that is no more than 20-25% lighter than stock.
Assuming the weight is removed more from the outer diameter, that will
translate into an effective weight reduction of about 50%. If you have tall
gears and/or a high lift cam with less low-end torque, then you won't want
to remove as much weight from the flywheel. Bear in mind that changing the
weight of the flywheel will not make much difference in overall performance.
The rule of thumb thrown around a lot is that one pound of flywheel weight
is the equivalent of ten pounds of chassis weight. You can do the math. In a
very closely competitive situation it might be worth an in-depth analysis to
find the optimum for a particular circumstance. Otherwise, take a
conservative approach and reduce the weight by only a modest amount.
I'm sure there are others that can offer more experience and knowledge than
I can. Try and locate some people who sell flywheels and ask their advice.
Here, for example, is a link to a source of aftermarket flywheels. I'm sure
you can find others.