Hello! That is a very good question, one that I can identify well with, as my last Omega Ruby team was 6 Megas in all. But, as the commentators above have pointed out, it depends on what your using it for. So let's start with In-Game.
In game is actually a pretty easy one to answer, and I will agree with DA830 on his advice: Give every possible Mega a Mega stone. This is because, since you have so many battles, you will be able to choose a Mega to super-effectively counter the AI. For example, if your oppponent is using primarily Dragon or Fighting Types, you would use Mega Salamence, as it all but obliterates those guys. But let's say they have a Fairy type as well? Then Mega Lucario can come in and Adaptability Flash Cannon/Bullet Punch/Whatever the heck it learns to the moon and back. And if you're looking for a good time, just use Mega Tyranitar, because Oh my Gosh is he chill. (Sarcasm, he's anything but chill, but he don't want to fight, if you know what I mean)
For Competitive, just forget everything you've ever learned, cause I'm about to tell you one word that will make everything else obsolete: Aerialate. Nothing else matters, just use Mega-Salamence.
But Actually Though: The great thing about these Megas is that they can fit in virtually any Competitive Team fairly well, and require very little support, with the exception of Mega T-tar, which may require a couple boosts from say like a Baton Passer, or for sure removing all Fighting types/Threats from your opponent's roster.
And that Concludes this answer, hope I could help :)