On a one-minute single-boss fight the build is 31/15/5. It goes 31 deep in Arcane for 
Arcane Power, then 15 into Fire for 
Pyroblast and 
Ignite, with 
Elemental Precision in Frost to make the Fire spells hit.
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0/1051205003100311531-035510001-005The search ran separately for every fight and faction, and the Arcane trees it found are almost the same. What follows applies to all of them. What changes is where the last twenty or so points go.
Why these talents
Missile Barrage tops the table at 22.0 % of your damage. A landed 
Arcane Blast has a 40 % chance, and a Frostbolt or Fireball a 20 % chance, to make your next 
Arcane Missiles free. That channel fires its five missiles every half second instead of every second. The single-boss rotations cast Arcane Missiles only when it is up, so it is also what spends most of your Arcane Blast stacks.
Arcane Blast is the spell itself. On this fight the rotation has nothing else to cast without it, so the sim reads its worth as your whole damage. Its job is to raise the next Arcane Missiles, Fire Blast or Pyroblast by 10 % a stack and to roll for Missile Barrage. Each stack adds 175 % to its cost.
Arcane Mind is worth 9.8 %. Forever rebuilt it into 10 % more Intellect and double the crit bonus for your Arcane spells. An Arcane crit now deals twice the hit instead of one and a half times.
Arcane Focus now gives 5 % hit with Arcane spells instead of reducing resists, and it is worth 5.6 %. Spells miss a raid boss 17 % of the time before hit, so even the fifth point is worth 8.6 DPS.
Arcane Instability and 
Arcane Impact are the crit and damage behind your spells, worth 4.8 % and 4.0 %. Arcane Impact used to be Improved Arcane Explosion. It now covers Arcane Blast and Arcane Missiles too.
Arcane Power adds 30 % damage and 30 % mana cost to every spell for 15 seconds, worth 7.0 %. It now needs 
Presence of Mind.
Incineration now reaches Arcane Blast and Fire Blast: 6 % more crit for three points, worth 2.4 %. Pyroblast (1.9 %) is cast once, made instant by Presence of Mind, and Ignite (1.3 %) adds 40 % of each Fire crit over four seconds.
Flexible points
Several points in every build do nothing in a boss fight, and they can go wherever your raid needs them.
- Improved Channeling. The spare points before Arcane Power go here, one to five depending on the fight, and they are worth 0.0 %. They give Arcane Missiles and Arcane Blast a chance to keep going when you take damage, and in our fights nothing hits the mage. If your raid takes area damage, this is where those points earn their keep. Arcane Geometry, Arcane Resilience, Arcane Shielding and Improved Counterspell do nothing for damage either.
Arcane Concentration on one boss is worth 0.1 %, because mana is never short there. You take it for Arcane Meditation, which needs it.
Arcane Subtlety. Two points with no effect in the sim, worth 0.0 %: the boss has no resistances for its 15 points of piercing to remove. Whether Forever turns piercing beyond a target's resistance into damage is still being tested in game, and our simulator doesn't model it.
Improved Fireball and 
Flame Throwing. Six points to reach Pyroblast. You never cast Fireball and range doesn't matter here, so both are worth 0.0 %.
What each talent is worth
A talent's worth here is what the sim loses when that talent's points move elsewhere, on the same simulated fights. Each talent is removed on its own, so the worths overlap and don't sum to your damage. Read the last-point column when the question is a single spare point.
On every fight
The builds share the Arcane tree and differ in the last twenty or so points. One and two minutes take Fire to Pyroblast and Ignite, plus Arcane Subtlety. Three minutes take Frost for Frostbolt instead. On three bosses for two and three minutes some Frost points go to 
Frost Channeling for Cone of Cold's cost. The one-minute three-boss build drops Missile Barrage altogether, since it never casts Arcane Missiles there. The table compares each fight's build with this one.