there is really no way to give an answer for this question that will work for everybody. Depends on what you weigh, how you ride, too many factors.
On my 421 cub, I ran 15-41
On my 10mil cub, I ran 17- 38
on my current K&T 4mil dragport, I am running stock gearing at 14-41
I have found that the stock gearing works really well for me, still fast on the drags, and will work in the hillshoots too.