They are all good.
Seriously, they all have good points and bad points and whatever works for you is best really.
The opposite foot forward makes room for a long stroke length, which can (provided the tracking remains true) increase overall acceleration which can produce more distance. It makes it easier to watch the backcast and fine tune it's path and timing.
*** It is essential to casting with a hard cross-wind with a single-hander, when casting literally backwards is the best method. ***
However it sometimes promotes "arm" casting and introduces tracking problems with so much freedom of movement along a long path. I find my body gets lazy this way and my arms do more work that my mid-section could be doing. Your results may vary.
In contrast the casting foot forward forces a more fixed and thus controlled stroke path. It sometimes promotes less smoothness in the backcast for those still learning until the caster learns how to apply smooth power to fully extend the line with a more compact motion, better yet to entend the body and arm in a Rajeff-esque way to maximize the back cast extension and then explode into the powerful forward stroke.
I always cast strong foot forward at the casting pool at fly shows
Both have their advantages / disadvantages, that being said while standing on smooth ground I prefer the casting foot forward except when dealing with cross-winds. Among other things I've learned to extend into the backcast and like the increased power up front to finish gained by recoiling the mid-body and rocking into the cast.
At the end of the day it's depends on the rocks at your feet. Learn to cast with your legs any which way and you'll catch more fish.