Reasons Why [Ooltha] Is The Best Contagion Zaw:
1. It only has a 0.05 lower Riven Disposition (1.30 vs 1.25), but significantly cheaper unrolled Rivens. As of patch 33.0, Kronsh Rivens are anywhere from 20-30 Platinum unrolled, while [Ooltha] Rivens are selling for under 5. (The [Rabvee] comes in at around 10 Platinum.)
2. It's useable as an actual melee weapon. The [Kronsh] and [Rabvee] have three options, Polearm, [Machete] and Hammer. Hammers are awful and Machetes are mid at best. While the [Kronsh] can turn into a Polearm, which is one of the better melee archetypes, it's primarily an Impact weapon. Being Impact weighted with very few forced Slash procs from Stance Mods causes this weapon to be a less than ideal melee weapon. On the other hand, the [Ooltha] is primarily Slash weighted and can be either a Staff or Sword. Swords, aren't great, but Staves are excellent.
3. It has better Stance Mods. [Clashing Forest] is great for building combo, so if you want to run a [Blood Rush] build instead of a [Sacrificial Steel] build, you now have that option.

Conclusion:
If you're building a Zaw purely for using Exodia Contagion, [Kronsh] is the way to go as it technically has the highest DPS potential. ([Shimmering Blight] is better for building combo compared to [Clashing Forest].) However, other factors like price and using the weapon outside of Exodia Contagion gives the [Ooltha] the edge over [Kronsh] (and [Rabvee]).

Link and Grip:
Technically, the Shutung and Vargeet II Ruhang provide the most damage, but there are a few considerations:
~First, you really shouldn't be using Exodia Contagion without Eclipse. Since Eclipse scales multiplicatively with the Contagion projectile, the difference in final damage with Shutung/Ruhang vs Seekalla/Jai is inconsequential as you'll be doing millions of damage regardless.
~Second, while the projectile itself isn't applied be base attack speed, how fast you can can spawn the projectile is affected by base attack speed because it is spawned by the swing of the blade. So having more attack speed is an overall DPS increase.

Riven:
A Riven is highly recommended especially because you can get them for essentially free, but if you really can't get a good one it can be replaced with either [Killing Blow] (if you intend to spam heavy attacks) or [Spoiled Strike].
The Riven on this build is the one I currently own, it is not optimal, melee damage would be preferred over Heat.