I take it as we speak about PvE, but i dunno btw. And for that i was keep asking myself this question too. I think simulationcraft numbers won't really matter because both the numbers can be changed throughout the expac and also raids lately barely include Pechwerk fights. So i think the best spec is the one which can perform good in the most scenarios possible.
Affliction +: You maintain much of your single target damage by your dots, you can even refresh many of them while moving. Best on council fights or fights with long living adds.
Affliciton -: While UA can be almost be called a direct damage spell as it has short duration it is best used on the same target more times so i would say it's not the best for burst addkilling (maybe with Haunt). Many of the Artifact traits focuses on killing a target, so while other artifact's traits can work in any case these may not in 1-target fights. Because of slow add-kiling it may not be the best in Mythic dungeons. Not much AoE burst. Decent one-target burst but not anytime you want it.
Demonology +: Also not bad on council fights maybe, because of increased SS regain. Also maintains much of its damage while moving (both DoT and demons), you can also chanell Demonwrath if there's nothing better to do while movement. Probably does the most AoE on long living targets with the proper talents. Probably does the most AoE burst with Implosion but it needs some setting up. OK cleave.
Demonolgy -: Currently pretty poor target swithing capability.
Destruction +: Best cleave. 3 Infernals with Artifact trait? Gets max DPS the fastest so i think it has the best switching capability. Simplicity makes you able to react to any situation fast whether you have Soul Shards ready or not. A good sustained AoE maxed from the first moment. Also very good on 2 target council or big add fights with Havoc.
Destruction -: Not that much thing to do while movement, though it will be a bit better with Artifact ability (which you probably not always time for movement). Damage comes more from direct spells so while you're not casting any you lose the most with this spec (gets better with increased Immolate damage talent). Lack of AoE bursts.
So if we want to decide i think we should check at the next tier's bosses instead of simcraft numbers. I really have no clue this expac what bosses will come (i always did with other ones) so i would give someone a cookie if he would count how many bosses have no/light/heavy movements or number of short living and long living targets or crucial DPS burst moments!
For me the most flexible seems to be Destruction. While i like depth in rotations its simplicity makes it good in any possible scenario. Also probably it will be the best for trash on Mythic dungeons because of how fast it goes to max DPS. The only situation where Destro may not perform the best is heavy movement little number of target fights i think.