This fight's build puts 18/33/0 into the three trees. It goes down Assassination to 
Cold Blood, a 16-point talent in Forever instead of 21, picking up 
Malice, 
Ruthlessness, 
Improved Slice and Dice and 
Relentless Strikes on the way. The rest goes down Combat to 
Adrenaline Rush, through 
Hack and Slash and 
Weapon Expertise.
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0/1005303103012-32520300001515231Every fight and faction gets its own search, but the builds differ only in points worth nothing against a boss.
Why these talents
Relentless Strikes refunds 25 energy after a finisher with a 20 % chance for each combo point it spent, Slice and Dice included, so every 5-point Eviscerate gets it. That one point is worth 5.1 %.
Improved Sinister Strike takes Sinister Strike from 45 energy to 40, an extra Sinister Strike for every eight you press. It is worth 4.8 %, and its second point alone 14 DPS.
Malice is 5 % crit, worth 4.1 %, and Forever extends it to your poisons. 
Precision is 3 % hit at 3/3, two ranks fewer than Classic's, and it counts for spell hit too (2.6 %).
Improved Slice and Dice makes Slice and Dice last 45 % longer, 13 seconds from one combo point, and Ruthlessness gives a finisher a 60 % chance to leave you a point. Together they let you open with a 1-point Slice and Dice and keep it up 91.5 % of the fight. The two add 3.3 % and 3.1 %.
Adrenaline Rush doubles your energy refill for 15 seconds on a 5-minute cooldown. You get it once a fight, so it is worth 8.5 % here and more the shorter the fight: 8.5 % on one minute.
Hack and Slash replaces Classic's four weapon specializations with a bonus for each weapon type, at 5/5: a sword or axe has a 5 % chance on a hit to make an extra main-hand attack, a dagger or fist weapon gets 5 % crit for its own hand, and a mace makes all your attacks ignore 15 % of the boss's armor. Rogues can wield one-handed axes in Forever. It is worth 2.6 %. In the pre-raid sets the Human's sword takes the extra attacks, the Undead's mace the armor, and both off-hand daggers the crit, which is why your weapons follow your race on the Gear page.
The rest of Combat adds to what you already press. Forever cut 
Dual Wield Specialization to 25 % more off-hand damage at 5/5, half of Classic's 50 %, though Wowhead's tooltip still shows the old figure. It is worth 2.7 %. Weapon Expertise takes 2 % off the boss's chance to dodge you (1.7 %), and Forever's new 
Flawless Execution cuts Eviscerate from 35 energy to 25 (2.5 %). 
Improved Eviscerate (20 %) and 
Aggression (6 % on Sinister Strike and Eviscerate) add 2.2 % and 1.8 %.
Blade Flurry is a different talent on one boss than on three. Alone, it is 20 % attack speed for 15 seconds every 2 minutes, worth 1.9 %. Against three bosses it also copies every melee hit to a second one, which makes it the build's most valuable talent at 15.9 %. Cold Blood makes one 5-point Eviscerate a certain crit every 3 minutes, once on every fight here, for 2.1 %.
Flexible points
Nine points buy little or nothing in a raid fight. They are there to reach the next row.
- Seven Combat points before Flawless Execution. The fourth row needs 15 points in the tree and the build wants only 8 in the first three, so the searches add
Lightning Reflexes and 
Puncturing Wounds, or Deflection, 
Endurance and 
Riposte. All of them read 0 (0.0 % for Lightning Reflexes), because they help only when you are hit or moving, or with Backstab. - Two Assassination points after Cold Blood. Most fights put them in
Improved Poisons, 2 % more chance to apply each poison a point. Some take one point of 
Vile Poisons (4 % poison damage) instead. Improved Poisons is worth 0.8 % here. 
Murder is 4 % more damage against Humanoids and Giants at 2/2 and can take the two Improved Poisons points. Forever took Dragonkin off it, and our boss is one, so the sim can't count it. Against a Humanoid or Giant boss it should beat the poison points.
What each talent is worth
Each value is the loss when the simulator replays the same fights with that talent's points spent elsewhere. The values overlap, so they don't sum to your damage. The last-point column is the one to read for a spare point.
On every fight
Every fight's build is 18/33/0 with the same core. Only the path points in Combat's first three rows and the two points after Cold Blood move. Below, every fight's build next to this one.