Rapiers have the highest damage multipliers on Finisher attacks, along with Forced Slash procs, making them great with Marked For Death.

[Dehtat] outperforms Kripath by a few % because Crit is additive with the Stealth Multiplier, it also offers better Riven Dispo.. not that you'll need one.

Riven stat priority: Finisher > Damage > Elemental > AS > CC/CD
The optimal combo is +Finisher +Damage -Harmless, because Damage is a fair bit better than Elemental.
Riven replaces one of the 90% Elementals, or PFury if it has AS.

Max Damage parts are chosen because Finishers scale better with base damage than with Crit. The loss in AS(Attack Speed) doesn't matter because Finishers don't scale with base AS, only with mods, up to +50%.

The math showing why this is the best combo:
Using a 49.2x multiplier on Rapiers, cuz that's the total final damage with the 24x direct (8x×3hits) and the 25.2x from the Slash DOT (4x×0.35×6 ×3hits).
For some weird reason the Slash procs use half the damage of the hits proccing them (4x vs 8x).

[Fragor Prime]: 62 208 = 270×24×(8+1+.4×(2.5−1))
[Arca Titron]: 79 834 = 360×24×(8+1+.24×(2−1))
DestrezaP: 80 629 = 170×49.2×(8+1+.32×(3−1))
[Endura]: 89 544 = 200×49.2×(8+1+.1×(2−1))
Kripath+Korb+V2R: 118 334 = (213+28+14)×49.2×(8+1+.36×(2.2−1))
Kripath+Korb+VR2: 123 719 = (213+28+28)×49.2×(8+1+.29×(2.2−1))
[Dehtat]+Korb+V2R: 121 973 = (224+28+14)×49.2×(8+1+.32×(2−1))
[Dehtat]+Korb+VR2: 127 428 = (224+28+28)×49.2×(8+1+.25×(2−1))

Crit is shit because it's additive to Stealth Multiplier. This means that it effectively has 1/8 = 12.5% of the effectiveness it has for normal attacks.
BloodRush+OrgShat: 1.38 = (8+1+.25×(1+4.4)×(2×(1+.9)−1))/(8+1+.25×(2−1))
2x90%Elementals: 1.4 = (1+(1.65+.9+.9+.9)×.5)/(1+(1.65+.9)×.5)
(assuming you're already using 165%Tox and 1×90%Elemental, and full 12x stack on BloodRush)

3rd 90%Elemental vs [Spoiled Strike](DPS) for Finisher damage
(assuming Arcane Fury, Primed Fury and 2×90%Elemental)
Spoiled Strike: 1.1 = ((1+1.65+1.8+1)×(1+.55−.2))/((1+1.65+1.8)×(1+.5))
90%Elemental: 1.17 = (1+(1.65+.9+.9+.9)×.5)/(1+(1.65+.9+.9)×.5)

Very strong combined with a Marked For Death+Fatal Teleport Ash build.