Melee Duplicate is ideal as long as you don't run Wrathful Advance. Other crit bonuses like [Zephyr], Avenger are okay since they increase the final average multiplier but reduce consistency.
If you run Wrathful Advance, swap Melee Duplicate for Melee Exposure.

Melee Duplicate + Subsumed Eclipse is approximately 20% better than Melee Exposure + Subsumed Wrathful Advance on all frames except Kullervo/Mirage.

Condition Overload is optional but is MULTIPLICATIVE on direct hits, so use this for Thrax/Demolysts specifically.

Primed Fury or attack speed increases projectile speed and initial throw animation.

Ideal Riven stats are DMG/CD/AS/TOX and a harmless negative e.g. -finisher. Replace [Gladiator Might].

Note, we use Seekalla to make it a Heavy Blade so it can use the [Rending Crane] stance. Currently, contagion inherits the damage multiplier of the first attack on the neutral combo of a stance. [Rending Crane]'s neutral attack has the highest first hit multiplier of 400%.

Use Corrosive for Grineer, Viral for Corrupted, Toxin for Corpus and swap banes accordingly
For Thrax, swap Primed Smite for Condition Overload (multiplicative on direct hits).

Corpus build: https://overframe.gg/build/241632/dokrahm/exodia-contagion-corpus/