As close to 12 o'clock and as far forward as you can comfortably use it is the best place for it. How to get it there depends on your rifle and what light you have.
Why 12 o'clock?
Curious as I prefer 6 0'Clock.
What TWR said.
The light will always be available even when bracing the rifle....Resting it on something or bracing it left or right.
There's less shadow on target with a light on top.
And it's ambidextrous, with a proper switch.
6 o'clock casts more shadow on the target and can get blocked by barricades / rests.