Define "best". For once I agree with both shoe and gil! Wow!
Motion by far the least. But not because of anything that stands out about motion itself. But instead because of what the other three offenses have to do with it.
If best means "volume", or if it means "I wanna shoot 3s all night and score 150 each game", well that would be fastbreak. 3 is more than 2. So running uptempo fastbreak, with a lot more 3pt shot attempts means a lot more possessions, means a lot more 3s shot/made if you have the shooters.
If best means total offensive functionally, I'll go with flex. Because flex it's more guard oriented. It's nice to have your SF spot to be a 3rd guard basically. Kind of a drive and dish offense. An athletic PG, with a Kyle Korver, Seth Curry and Dirk on the perimeter, it's gonna rain 3s all night.
If best means, offensive programming design, I go triangle. I've always felt like the most deadly weapon in HD as a stand alone, is a 100 PER shooter in triangle. It relies most on the LP/PER ratings for scoring. Even if the ATH/SPD/BH/ST/etc. might not be ideal. You can also load up the distro on a scorer like this a bit better than you can in other sets.
so your definition of "best" is what's more important to answer your question. And as we all mentioned, motion is last. It's better to get all your players involved in the scoring with motion. That means getting your bigs involved, which to me means it can't be the best